Bogen K10 Amp

[DummyLoad]

interesting little amp - 10WRMS using 6AU6 1st stage, 12AX7 PI, and 6AQ5/6005 pentodes in PP, w/ 6BW4 recto. i'm throwing darts, at about 250-300V B+ at the plates. i have fired it up - it hummed fairly loud for a bogen, so i shut it down before i could get telemetry.  

interesting, B/C i've not seen such a simple 10W amp that seems so complex.

- C8,C7 and R10 - no clue as to what that network is for...
- the tone stack for little 10W seems too complex - i don't think they weren't pinching pennies on this one.
- paraphase PI - ho-hum... :\   but it does get more interesting...  
- tone stack is in the F/B network of the non-inv. stage of PI.
- the 330ohm (R8) seems odd - seems like the R9-R8 junction is where the NFB tap would be?
and not the cathode of V2b...  
- 6AU6 1st stage is interesting, PRR 1/3W comes to mind. - why the R3-R4 divider for the G2?
- cathode biased output stage... but with a strange twist...
- 68K 2W resistor from G2 B+ to cathode of output stage.
- 33ohm virtual CT for fils. - they are 1/2W and both burned to a crisp. i guess somebody missed the mark on the P_diss of those resistors.

none of the tubes test bad, and the iron seems good - e.g. not tarry or burned, however, all the filters are shot, and some of the wax coupling caps are falling apart.

the amp does not appear to have ever been repaired, i think i was the first guy to crack the bottom off of it. i did break a cap off one of the volume pots poking around...  

it was mounted onto a 19" rack panel that looks like it was ordered that way as an option.

ill get some gear porn (gut shots) posted up tomorrow - i'm too tired tonight.

.SCH and .PDF attached...      
 

[PRR]

There is a ground where VR1 and VR2 come together.

It is unlikely R23 is 270K.

> - C8,C7 and R10 - no clue as to what that network is for...

It's a mystery. Heavy-NFB amps often need some hundred-pFd cap to kill supersonic gain before the OT resonances skrew-up the phase.

> - the tone stack for little 10W seems too complex - i don't think they weren't pinching pennies on this one.

One switch, one knob. This is a low-price-point product. Yes, it may have cost a dime more than a James. However a James with standard pots will need more gain.

> - tone stack is in the F/B network of the non-inv. stage of PI.

It uses the excess gain of the power section. Doing NFB tone-control here needs reverse-taper pots and other troubles.

> - the 330ohm (R8) seems odd - seems like the R9-R8 junction is where the NFB tap would be? and not the cathode of V2b... 

C5 makes both points the same for audio. DC level might pop, and might not, and who cares? I guess they flipped a coin.

> - 6AU6 1st stage  - why the R3-R4 divider for the G2?

More interesting is the grid-leak bias at G1. Grid leakage is rarely quantified, so no analysis is possible, it was designed on the breadboard.

The maximum output from a tube is almost half the plate supply voltage. The maximum input is the max out divided by gain. In a pentode, it is also usually the voltage on G2 divided by the Mu(g2) of the pentode. Ipso facto, lower G2 voltage leads to higher voltage gain. Also we can approximate the maximum input voltage from G2 voltage and Mu(g2). If G1 bias were known, assuming 200V supply to this stage, we get roughly 50V at G2 and Mu(g2) of 6AU6 is 36, so max input is ~~1.3V peak. Ample headroom for PA microphone. Barely ample for guitar. And due to grid-leak uncertainty, the actual max input must be found by test.
> - cathode biased output stage... but with a strange twist... - 68K 2W resistor from G2 B+ to cathode of output stage.

The 4mA here is not dropping G2 voltage nor raising cathode voltage enough to note. It's not bias. Looking around, there is no other bleeder. While the main caps will fall at turn-off, they won't go to zero by the time the cathodes cool and quit emitting electrons. 45uFd(total) against 68K is a time-constant of 3 seconds. In 3 seconds the B+ drops to 37%; in 15 seconds it drops to 1%.

> - 33ohm virtual CT for fils. - they are 1/2W and both burned to a crisp. i guess somebody missed the mark on the Pdiss of those resistors.

No. 3.15V across 33 ohms is 0.3W, and nominally safe. They may drift, but won't crisp. However if one got shorted, the other would get 6.3V, 1.2W, and might crisp. Or if a power tube developed a heater-cathode short, the 330 ohm bias becomes less than 33 ohms, tubes attemp to flow "infinite" current, and in this amp that just -might- be enough to burn these resistors.

As a gitar amp: the gain is high but not wrong, the bass-response may be "too good", the power section is too clean. Disconnect flavor-switch, take 15K from 16 ohm tap to X. For a lighter (less bass) tone, decrease C2. If gain is just too darn much, connect V1 as triode, or put some resistance to ground on the mix-network.

[DummyLoad]

There is a ground where VR1 and VR2 come together.

yes, it was late; i overlooked in the transposition from hand drawings to .SCH

much thanks for the analysis, it really clarifies things a lot.

thinking i'd like to hack out the phono crap and install simple valco type tone control.

...maybe also turn selector into NFB level switch or pot. would 10k fixed + 10k lin pot be a good starting point instead of 15k fixed value for NFB you recommended?

[DummyLoad]

The maximum output from a tube is almost half the plate supply voltage. The maximum input is the max out divided by gain. In a pentode, it is also usually the voltage on G2 divided by the Mu(g2) of the pentode. Ipso facto, lower G2 voltage leads to higher voltage gain. Also we can approximate the maximum input voltage from G2 voltage and Mu(g2). If G1 bias were known, assuming 200V supply to this stage, we get roughly 50V at G2 and Mu(g2) of 6AU6 is 36, so max input is ~~1.3V peak. Ample headroom for PA microphone. Barely ample for guitar. And due to grid-leak uncertainty, the actual max input must be found by test.

so then, if 68k were increased to 91k, with Va @ 200V then we'd see ~82V @ G2  and ~2.2V headroom? is that a be a better starting point for guitar? meanwhile, resistors are cheap and it's a weekend. i have some hacking to do.

 

[DummyLoad]

- tone stack is in the F/B network of the non-inv. stage of PI.

i think i made a mistake, it's the inverting side of PI.

[PRR]

> it's the inverting side of PI.

There is no way to know from inspection. Polarity of the OT is not known.

However it is surely NFB. A PA amp must have low output impedance. Say it fed 10 classrooms at 1W each. And say that system gain/volume is correct when all 10 classrooms are connected. Say the principal is yacking, and 9 teachers decide to cut their speakers. The volume on that 10th speaker should not rise enough to notice. You want a good Damping Factor. 3 is poor, 10 is good. To get this with tubes, you must apply NFB at the output.

Same is true for most Hi-Fi (including simple one-room PA) thought since the 1940s. Amplifier has low-low output impedance, speakers are designed for that.

(When it is happy) bring it up with 1V across a dummy speaker. Disconnect the dummy load. If the output is now 1.1V, damping factor is near 10, 1.2V is DF=5, 2V DF=1.

While a faithful versatile PA amp has high damping, guitar amps and their speakers are often worked with very little damping. My suggested NFB resistor gives roughly unity damping, a compromise between dull and over-excited. If you go down to 2K it will be "very (too?) clean"; you might get oscillation below 1K. If you go over 30K to loosen it up, you may as well disconnect the NFB.

So given a switch, and an exponential scale from 2K to 30K, you might switch resistors like 2 4 8 16 32; or six handy-values like 1K8 3K3 6K8 10K 15K 33K. I'd just tack-solder... after use (with both open- and closed-back speakers), you may decide to have more steps in some range and skip others.

> hack out the phono crap

Label it "iPod". It is perfect for that (and will NOT work with modern mag-phono). Use stereo-RCA jacks, mix L and R with two 10K resistors to the Phono pot.

I think the 6AU6 preamp is good the way it is. Don't tamper/tinker until you beat on it awhile. Sure, if you want a different vol-tone affair, try it.

[DummyLoad]

It is unlikely R23 is 270K.

missed that one.. yup, it's 2.7k  

thanks for additional info, i'll leave 6AU6 alone for now then.

why would i want ipod input w/ mono geetar amp?  more useful as tone ckt. so it gets hacked into a tone ckt.

switch gets whacked, pot and single resistor are a simpler solution.

[DummyLoad]

ugh!~ i left out 4 components... i guess i forgot to go back to the OT when hand drafting the schema.

added:
- R-C network (R34 & C17) across OT pri. CT and one plate leg.
- slug cap (C10), on V4 from g1 to k.
- 10k 1W (R33), from OT sec. 500ohm tap to gnd.

fixed:
- R23 value
- omitted gnd @ VR1 & VR2

attached updated schemas.

[DummyLoad]

thinking about a mod for more V1 gain. after reading one of PRR's previous posts in this thread, i came up with the scheme as shown in the attached.

please see .sch and .pdf files attached.

feasible? will it work, or will it leak smoke? regardless, we'll know tomorrow... 

[PRR]

Won't work without either the original grid-cap, or a cathode-bias scheme.

Well, it will "work", but overload on soft strums, without making much output.

Re-study "grid leak bias". It works, up to a point, but you need the grid cap.

And the grid-cap really should be larger than 0.03u. That's Bogen price-pointing this cheap model; their higher-price models may use 0.1u and overload more gracefully. For guitar, that's a matter of taste, but 0.03u is probably on the lean end of "tasty". 0.5u may faint after plucks and recover too slowly, the fat end of tasty. Tweak to ear.

For modern guitar, and no large lust for old-tyme design, I'd just stuff 1K-2K cathode resistor in there, with your favorite preamp cathode cap. Then the grid can be 1Meg plus the guitar.

In some venues you will want ~~100K series grid resistor to snuff AM radio. Yes, larger than the 34K we use on triodes because pentode has low grid capacitance.

As for your switch: may not make any major difference. Or not that you can't compensate with string-touch and volume-knob adjustment.

Try varying G2 from 10V to 100V. You may need to fiddle a cathode resistor to keep plate in the 1/3rd-2/3rd of B+ zone; or you may find that letting plate get far off the mid-point of B+ gives you an asymmetric tone you like.

[DummyLoad]

Won't work without either the original grid-cap, or a cathode-bias scheme.


funny you should mention that... i fired it up and learned that lesson about an hour ago. was wondering why volume pot on geetar was scratchy - duh! DC on the pot.

i'm tearing down V1 to make K bias stage.

figures i'd learn the hard way... please pass me the dunce cap... again!   

I'd just stuff 1K-2K cathode resistor in there, ...

i was thinking a happy medium of 1.5k to start with.

In some venues you will want ~~100K ... 

OK, will do.
 
As for your switch: ...

i didn't add it, after some thought, it seemed like too much of a hair-brained scheme.

i took some measurements:

A = 323V
B = 312V
C = 272V
D = 173V

V2a bias = 1.62V
V2b bias = 1.62V

V4,5 bias =  22.2V

it does sag nice... point A dips down to 280V or so on DMM... hard to tell exact peak of dip, DMM sampling interval.   

 as always, thank you for the education! :-)

[DummyLoad]

well, this turned out sort of nightmare-ish... i was reconfiguring V1 and broke a pin, OK, no problem, i'll pull it and replace it... pushing new pin in breaks socket. heat fatigued from all the soldering and component swapping. R&R old socket w/ dremel tool to grind down rivets while cussing self for being such a gorilla.

why 1k for Rk? - you gave me a range - all i had on hand (errr.. could find?) were 1k and 2k7.  .047uF Rg because i butchered the last .1uF i had - .0047uF coupling because in the last variant .022uF was a too boomy.

what i ended up with was well worth the effort.  this thing is now pure evil but sweet when i back off on the controls. wide open is much, much more fun. amp is connected to a JBL K120, w/ open back cabinet.

i have not added NFB yet. it would probably have helped some with the boomy, but now as it is, it's near perfect for what it does.


i attached a schema of what the final version looks like.  .sch and .pdf file formats attached.

the telemetry may have changed, i'll update tomorrow.

thanks again for the guidance - that was a fun hack.   

[DummyLoad]

this amp is done... i couldn't stand to leave a hole in the chassis, so i added a presence control. it works nice. dial down for clean playing - dime it for wild.

schema in PDF. & .SHC file formats attached.

[RicharD]

I got to take this little beast for a test drive last night.  Wolf in sheep's clothing it is.  All those little tubes (6AQ5's and 6AU6) put fourth a lot of sound.  I was very impressed and didn't wanna stop playing.

[DummyLoad]

K10 found a new home...

sold it for 50 bucks and an AC/DC concert ticket...  :glasses9:

new owner is...

[RicharD]

 :walk:

[dyerjol]

New to the forum, sorry to revive this old thread, but I just gotta. 

I used the Hack 1.3 schematic as a basis for rebuilding a Bogen K10, and I thought I should throw in my 2 cents in case anyone else wants to try it.
There are a couple of mistakes in the Hack 1.3 schematic. There are two resistors marked R4, R32 is skipped, and two capacitors marked C2. Not a big deal, except when ordering parts.

FYI, this is what I changed:
Instead of a tone pot, I used a Matchless style bass contour switch.
Instead of variable NFB, I made it switchable.
Added a Matchless style treble cut pot.
Added a VVR.
Various component value changes to personal taste.

It sounds amazing! Big thanks to DummyLoad for the schematic!

[Ed_Chambley]

Bump.

[sluckey]

bump bump bump???

[Ed_Chambley]

bump bump bump???

Got a PM asking about this amp and wanting a schematic.  I have no need personally, but thanks for asking.

[Platefire]

It appears to be open season on old Bogens. When they were putting out all these PA's/Intercom systems, they had no idea they would latter become a guitar amp hackers dream

[DummyLoad]

Bump.

do da bump bump bump... oh, so sorry. back on track. 


schematic for k-10-schema - plate also posted the reverse engineered copy - it's the K-10 before i hacked it.


schema's attached: r 1.3 is the first one i built in august 2009. it sounds the best, r 1.5 is a 2nd unit i built last year and this r 1.5 doesn't sound as good as the r 1.3. i fixed a couple of minor ref. designation errors in the schematic that was originally posted. owner plays the r 1.3 mostly with the NFB out.


owner of r 1.3 brought it over for a side by side with the r 1.5 last year. the paraphase is the magic. oh, and the speaker: we played both through a pair of UK made celestion G12M in semi closed-back pine cabinet.


--pete

[Paul1453]

Thanks for bumping this back up guys.

I just got one of these and I saw the Bogen 20 thread, then I bothered Ed (since he has been very helpful to me in the past with obscure schemos) before I bothered to search the archives.  Sorry about that.   

Thank you very much DummyLoad for posting what you have on this, and letting me know which version of modifications you have personally taste tested and prefer!
Extremely helpful information!   

Here I was all excited about the Tone Stack from Hell rotary switch, and then I learn it is one of the 1st things to go.   
I thought that was gonna be a real cool feature of this amp, but I guess not.

I also noticed we go through the 6AU6 in the preferred model.  I was just going to switch out the phono input and try to leave the mic option available.
I suspect you have already tried that and decided your modification is better.  I do have a premium Mullard 6AU6 to put in this and a Bugleboy 12AX7.   

Obviously I'm going to implement your preferred design as SOP.
I guess I even get to reclaim some unused parts.  Mic transformer and tone switch from hell?

I'm very much looking forward to getting my K10 and transforming it into a great amp!   

[DummyLoad]

> Here I was all excited about the Tone Stack from Hell rotary switch, and then I learn it is one of the 1st things to go.  I thought that was gonna be a real cool feature of this amp, but I guess not.

give it a go first before you gut it. you MAY just like it as is. i did not, but that that's my taste: your's is certainly different. i have left a few of the PA amps i've built mostly intact, e.g. simple re-tuning inter-stage roll-off points or tweaking/changing the tone stack is usually all that's needed. my taste said overhaul as the K10 as built it was too dark. the pentode got some attention and the switched TS was replaced variable and switched NFB was added later.

another gent used the tone selector switch for a matchless style bass roll-off selector on his.

-pete

[Paul1453]

TSFH (Tone Stack Fom Hell) is the connection of the NFB loop from the OT.  From what I am learning and experiencing, in guitar amps we only want a very limited or what I seem to prefer no NFB loop connection.  We certainly do not want the type of negative feedback as designed into PA systems.

If I want to overdrive my output tubes, I certainly don't want something built into the system that once I do start to overdrive it feeds back to the input and impedes or eliminates my ability to overdrive my output tubes.  I can see this design as it applies to Hi-Fi or Public Address systems, but IMO it is the antithesis of the guitar tube amp.

We want to be able to overdrive the output tubes and to control the amount of overdrive ourself and not have a feedback circuit try to do it for us.
Maybe just a slight amount of automatic negative feedback control to keep our circuit from oscillating wildly out of control, but no more than that.   

So am I wrong to assume that unless the TSFH is eliminated, I will find it has far more NFB loop control over my output tubes than I want it to have?

[Willabe]

So am I wrong to assume that unless the TSFH is eliminated, I will find it has far more NFB loop control over my output tubes than I want it to have?

Maybe, maybe not.

1st, just disconnect the NFB wire, won't hurt anything, 2nd, then you can use a pot instead of the NFB R that goes from the speaker tap to the NFB injection point.

Play with that to dial in the amount of NFB you like. 

[Paul1453]

So on the original K10 design, you are saying to disconnect point 2?

Or insert say a 1M pot between R23 and point 2?

Or just replace R23 with a 1M pot?

[Willabe]

I haven't looked at the schematic, but the NFB is taped off the OT's secondary. Then there's a resistor that is in series coming from the OT secondary going to the NFB injection point/connection. That R replace with a pot.

[Paul1453]

Yup, on the original K10 design R23 is that resistor that taps the OT's feedback through the TSFH to get to the NFB loop insertion point X.

So in my case, I want to reduce or eliminate the NFB loop's ability to stop me from overdriving the output tubes.

That means I would not want to lower R23's resistance any more than it already is.  I would want to increase R23's resistance to the point where I'm happy with it's effect on my ability to overdrive the output tubes.

So to achieve my desired results, it would be best and easiest for me to lift one leg of R23 connect that to my 1M pot and then connect the wiper's output to the point where I lifted R23's leg.

I can then adjust my pot's resistance from 0, like it is not even there and is functioning the same as the original design, up to the full 1M resistance of my pot.
I can then increase the resistance my pot adds to the circuit, from 0 to the point where I am happy with the amount of NFB it can deliver to the insertion point.

The only downside of this I see, is that this reduction of NFB also reduces my TSFH's ability to shape the tone.  If I reduce the NFB to 0 I also reduce the tone shaping aspect of my TSFH to 0.  I guess those are the pro's and con's of this modification.   

[jazbo8]

IMO, a tone stack designed for guitar amps would be more appropriate - you don't have to deal with the NFB network from the K10 at all, or if you must have feedback, simply install a simple presence control.

[Paul1453]

I was considering just following DummyLoad's r1.3 design, but he suggested I give the NFB loop and TSFH a try before I drop it.

I'll probably try adding the 1M pot as I outlined before going to the r1.3 design.

Since my preference is to have little or no NFB, I'll probably end up using that anyway. 
At least I will have experienced the TSFH for myself if I follow his advice. 

[DummyLoad]

i was trying to point out to just try the amp as-is before hacking it; that's all. yes, it has a unique tone control, but if you go hacking at the NFB circuit, it's very likely not going to produce favorable results.


--pete

[Paul1453]

i was trying to point out to just try the amp as-is before hacking it; that's all. yes, it has a unique tone control, but if you go hacking at the NFB circuit, it's very likely not going to produce favorable results.

--pete

No plans on hacking on the NFB or TSFH on my own.  I have a few questions, if you don't mind.

V1.1 is updated stock, where I should be starting from.  The plan to start with was:
Take out phono jack replace with 1/4" jack and use this as the new guitar input.
Replace one of the current speaker out jacks with 1/4" jack to plug in 1 12" 8ohm speaker to start with.
Like this a XLR mic could still be plugged in and tested and I can twiddle with the TSFH's switch to experience all it's flavors for myself.

V1.2 has the TSFH eliminated and no NFB.

V1.2 + V1.3 both have 1/4" jack on the input of the 6AU6, eliminating both Mic and Phono inputs.
C2 changes to C3 and goes from 100n to 47n and an e cap C1 47u 25V gets added.
What was a vol pot becomes a tone pot and R7 and C4 get added in there.

V1.3 replaces the TSFH with a switch and Presence pot.
You mounted the Presence pot where the TSFH rotary switch was? 
Where did the NFB loop cut switch go?
In the hole left from the phono or mic jack?
No one has kept the Mic input?
Do we want the extra gain that 6AU6 can give us?

Like I said, I plan to start with just a new 1/4" input jack at the phono point and a 1/4" jack for the speaker out.
Then play around with it like that for a while, before on to a later version.
I'll call my minimally changed version V1.1m for still having a Mic input.

Any other comments or suggestions?

[DummyLoad]

other than making sure that the power supply filters aren't toast, adding the 1/4" in/out jacks are where i'd start.


--pete

[Paul1453]

other than making sure that the power supply filters aren't toast, adding the 1/4" in/out jacks are where i'd start.

--pete

Replacing this one before I plug it in.

[DummyLoad]

yes, do!


--pete

[Paul1453]

yes, do!


--pete

It's not a 30u 500V, I've got 22u 450V in stock.  Mine has some 500V and 600V caps in it (overkill).
I don't think this 22u 450V cap should cause any issues.
I thought I saw some V readings as like max 325V B+?
I notice the revision versions all say 400V caps.

Starting hacking this when I get off work tonight.    

[DummyLoad]

yes, do!


--pete

It's not a 30u 500V, I've got 22u 450V in stock.  Mine has some 500V and 600V caps in it (overkill).
I don't think this 22u 450V cap should cause any issues.
I thought I saw some V readings as like max 325V B+?
I notice the revision versions all say 400V caps.

Starting hacking this when I get off work tonight.   

400V as stated for the coupling caps that i used Unless Otherwise Noted. electrolytic too, however 350V and 450V electrolytic are more common.

there is no way a 6aq5 amp will need 450V caps. if that's all you have then use the 22uF 450V. it wants 30uF @ 350V. so you could parallel 2 x 22uF and run 44uF without incident. in rev 1.3 the replacement can i used was a 40/10/10 @ 450V.

i cleaned up the r1.3 schematic. parts are more legible.

--pete

[Paul1453]

Changed that cap, put in the jacks, and gave her a go.

TSFH has some effect, but not as much as I thought there might be.
My main complaint about this amp as it is now is, it doesn't seem to have 10W of output.
It seems to be less loud than my 12AX7 6V6 SE Champ clone.

I first tried it with 1 12" 8 ohm speaker.  Then 2 12" 4 ohm speakers.  I did change the jumper on the OT when I did that.
Not noticeably louder with 2 speakers.  Checked Voltages, all at or above what is showing on v1.3a schematic.
Swapped output tubes, the 12AX7, even the unused (at this time) 6AU6.  Many different combos, no change.
I expected it to be significantly louder than my other SE 6V6 or 6AQ5 amps, but it is not.

One thing I did notice is that the output drops to 0 with the volume knob at 5 not 0.
There is no hum, it doesn't sound distorted or anything else.  It sounds quite good, but weak.
It doesn't seem to overdrive at all.  I haven't tried adding gain and distortion on the input yet.
Is that NFB loop doing this?  I wouldn't expect it to be this big of a factor.
I can't seem to push this amp at all in this configuration, and I don't like only having half a volume control.  silent at 5 instead of 0.

I'm going to put a distortion box and add some gain to the input and see how it responds to that.
Something doesn't seem right as it is now.  It seems weaker than a SE 6AQ5 amp, and that's not right. 

I checked that repair I noted in inspection.  It was C12 on v1.3a 10u 50V e cap. 
Replaced with 10u 68V e cap, just because this is what had been changed before, this made no change.
My Voltages are: A +344  B +332  C +296
v1.3a shows:       A +323  B +312  C +272
Voltage on + end of C12 +23.3  that is the suspect cap I changed.  R18 measures 333 ohms.
I'm not finding anything that is way off.  The sound is very good, clean, no distortion or hum, responds to the tone switch.
OK, time to hit the input with some gain and distortion.   

The NFB loop seems to eat that right up.   
Back to 8 ohms and 1 12" speaker, seems a little louder than 4 ohms and 2 speakers.   

[sluckey]

That phono input wants to see a much larger signal than a guitar can produce. You must plug into the mic jack for the extra gain needed by the weak guitar signal.

[Paul1453]

That phono input wants to see a much larger signal than a guitar can produce. You must plug into the mic jack for the extra gain needed by the weak guitar signal.

I had asked about that, but didn't get the right answer until now.  Thank you.

Maybe dummyload's K10 didn't have the same XLR mic input as mine?
Mine goes to a plug for an input transformer.  His v1.1 schemo doesn't show any of that circuitry.
Without that gain from the 6AU6, this amp's performance is quite unsatisfying. 
Leaving the NFB as designed keeps the amp in pedestrian mode even when you pump up the input with an external preamp.

He and his buddy seemed very pleased with the v1.3 hack.
That is kinda where I thought I would end up.  Moving on to v1.3
Need to remove some parts (also find the added parts and rewire as in v1.3)

Any problems with v1.3 's schemo, or other issues I should be aware of?

[sluckey]

All you need to do at this point is rewire the grid and cathode of the 6AU6 IAW the schematic in reply #37. Leave everything else original. Then you can properly evaluate the amp to see if it needs further mods.

[Paul1453]

There are also a number of changes between V1 & V2.

The stock NFB loop just doesn't do it for me.  I already gave that a try.
It just sucked up all the external preamps gain when I applied it,
I don't want a huge governor on my hot rod go-cart's engine.
It's also a lot easier to work on if I get the unused crap out of the way.
This had the cap can cut out and left in place and new e caps built on top (before I got it) so there is some extra clutter in there to deal with.
I can just switch the leads from the jack I've already got wired to V1 and continue to follow v1.3 from there.

[Paul1453]

On pin 7 of the 6AU6 is a 47u 25V e cap.  Does this need to be 25V? 
I found a 47u 10V e cap in my little e cap bin.  I haven't checked all my other junk gear for a 25V one yet. 
Is 10V too small and I need to get 25V?

I also have a 22u 25V if that works?

[DummyLoad]

either will work. 10V or 25V part.


--pete

[Paul1453]

Now that's what I'm talking about.  v1.3 ROCKS!   

I didn't put the switch and presence pot in yet, and she's putting out 10W now!   

[DummyLoad]

glad you like it. don't bother with v1.5. you'll not be happy. add the presence control so you can tighten up the  bottom end some.

--pete

[Paul1453]

glad you like it. don't bother with v1.5. you'll not be happy. add the presence control so you can tighten up the  bottom end some.

--pete

Mine had a 4.7K NFB resistor coming off the OT.  I put in a 25K pot where the rotary switch was. 
Left the switch out because I didn't really have a good spot to put it.  My pot was actually like 28.3K or so.
Ended up maxing the pot before I really liked the amount of feedback.  It's OK maxed, I guess.   

Your friend usually runs his switched off?

I'm considering using like a 100K pot, finding the amount of NFB I like, reading that value, and then just putting in a fixed resistor at that value and the switch.
No adjustment then.  Very slight NFB at the amount I like, or switched off for no NFB.   

[Willabe]

Try it.      (Won't hurt anything.) 

[Paul1453]

Any idea what amount of resistance added there is effectively no NFB?

If I used a 500K pot instead, would the NFB effectively be off, oh just a wild guess at say anywhere above 250K?