Recent Posts

61

Tube Amp Building - Tweaks - Repairs / Re: Valco National 1201 1202 Sportsman

« Last post by passaloutre on June 04, 2026, 12:24:20 pm »

I'm with you on the 6SL7! Though lately I've been using the "Loctal" 7F7 version, converting a couple old PA/HiFi amps to Tweed Deluxe variants with the full suite of Loctal 7F7-7C5-7Z4. As far as I can tell the 7F7 internals are identical to a 6SL7, but they are available NOS or JPO (just-plain-old) for pennies.

Anyways, maybe its psychological but these octal/loctal preamp triodes do sound sweeter to me

62

Tube Amp Building - Tweaks - Repairs / Re: Ch. Mixing resistors of 100K, 220K, 270K or 470K?

« Last post by stratomaster on June 04, 2026, 12:15:35 pm »

Everything is a filter (especially when adding caps in parallel with the mix resistors) and everything is a voltage divider.

While the divider ratio is the same between these, the overall resistance (impedance if the coupling cap is small enough) to AC ground is significantly different between the different values. You'll need to trace the circuit back through the coupling cap and plate resistor to HT, then parallel that with the functional triode residence to get a good calculation

As to how they'll behave, that depends a lot on the other values in the circuit.  In general, you'll have marginally more signal the higher you go, but there's also a voltage drop across the first mixing resistor, so there is a ceiling. There are also noise considerations as higher resistances are inherently noisier.

63

Tube Amp Building - Tweaks - Repairs / Ch. Mixing resistors of 100K, 220K, 270K or 470K?

« Last post by Brownie on June 04, 2026, 11:44:36 am »

Can anyone reveal the ways the various values of mixing resistors behave in the circuit? This early Traynor had 100K ones, but I substituted 270K ones, split the V1 cathodes and put a bunch of Marshall values into it for buddy client. That amp is finished and now I'm doing up my 5f2/5f3 Frankenbeastie with a stacked tone pot. What should I choose for those mixing resistors? Maybe I'll use 220K like on a Fender AB763

64

Tube Amp Building - Tweaks - Repairs / Re: Valco National 1201 1202 Sportsman

« Last post by bmccowan on June 04, 2026, 11:34:06 am »

I have to say that amp looks super cool - if I had an original one, I would definitely try to keep the circuit intact. But for building one based on it, I'd be inclined to do a single channel and cascade the two 6sl7 triodes. Would that be too much gain?

Hey Dogburn - That is what I suggested to Roarshock. Lots of amps are designed that way, so no, not too much gain IMO. If it is, easily adjusted with the preamp plate resistors and the bypassed cathode resistors and caps. I love the clean sound of 6SL7s. I know the specs are similar to a 12AY7, but somehow they are just sweeter. Could be my imagination, no blind tests here.
I built a single ended "1947" Valco that is 5Y3/6SL7/6V6 with a 10" speaker - its a sweet thing.

65

Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by kagliostro on June 04, 2026, 09:59:40 am »

To test if it works you can use the preamp of one other amp or a signal generator or arrange a partition resistor from the output of a small transistor radio

Franco

66

Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by wsscott on June 04, 2026, 09:53:12 am »

Assume the attached Gretsch schematic of the 6165's power amp has the AC power line and fuse attached at the PT's primary windings.  Other than that, no controls and nothing from the Preamp is connected.

67

Tube Amp Building - Tweaks - Repairs / Re: silvertone 1484

« Last post by passaloutre on June 04, 2026, 09:45:34 am »

I'm not saying that. I'm just discussing the differences in the circuits. It's up to you which one you want to build.

If you want to stay true to the original, Mercury offers a "ToneClone" power transformer here, for $200: https://www.mercurymagnetics.com/model/1484/. Looks like it has two 91V taps for the voltage doublers, and another 26V tap for the bias supply.

Otherwise you can use a more traditional power supply, and choose to keep the screen voltage high (as Mojotone did), or find a another means to drop the screen voltage.

For the record, here is the original schematic: https://el34world.com/charts/Schematics/files/Silvertone/Silvertone1484.pdf

68

Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by wsscott on June 04, 2026, 09:37:07 am »

Thanks Shooter.

69

Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by shooter on June 04, 2026, 08:52:43 am »

wondered if there is a way to test it.

sure, no problem, just provide the following;


total expected;
 filament current/voltage
 preamp current/voltage
 PA current/voltage

then you can calculate a dummyload for testing

70

Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by sluckey on June 04, 2026, 08:23:21 am »

How can you continue to ask for help on this project when you won't share the schematic? I understand honoring someone's request to keep the secret, but if everyone honored that secret then you wouldn't have the schematic. Think about it!