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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, 07:14:20 am »

I've almost completed building the Power Amp section, and wondered if there is a way to test it. This section is where the AC comes in and contains the PT, OT, 2 chokes, and the rectifier and 2 6L6's.  The on/off switch is in the preamp, not this section. Since I haven't started construction of the Preamp, I don't have a way to inject a signal, or anything else, but I want to make sure this section of the amp is wired properly.

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

« Last post by johnfromcyrene on June 04, 2026, 01:09:19 am »

Are you saying just to use a different power transformer then? change that and nothing else?

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

« Last post by dogburn on June 03, 2026, 09:39:17 pm »

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?

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

« Last post by bmccowan on June 03, 2026, 09:19:58 pm »

Happy to share and happy if it ends up in the library. Some other oddballs I've worked on have ended up there.
Sharing is what the Forum is all about.
Funny - I have seen the 1201 and 1202 models numbers attached to a single ended 6v6 1x8, a parallel SE 2x6v6, the circuit shown here, and a 2x6L6 with 2x10" speakers. It's like they could only count to 2.
The instrument channel lacks both gain and volume compare with the mic volume. I think going with a different T&B tone stack or increasing the resistance of the 10K tail resistor could go a long way toward bringing that channel out of puberty.
I think there is no real magic in this circuit, other than the use of the 6SL7 in V1. It's my favorite preamp tube.   

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

« Last post by roarshock on June 03, 2026, 07:19:59 pm »

Thank you for sharing your work!
Would you be alright if it ended up in the schematic library here? After my hunt for it, it'd be cool if others could build (or repair) their dreams. You were right that National played a little loose with their numbers. The insides of the only 1202 I could see showed 100k pots. Having a starting point like this for a build is wildly helpful.

Is the instrument channel just to clean for your liking or is it also significantly quieter than the mic channel too?

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

« Last post by passaloutre on June 03, 2026, 04:48:51 pm »

Yes! I think it's not just the overall B+ plate voltage, which shouldn't be that different between the original Silvertone and the Mojotone (the Silvertone schematic says 475 and my Mojotone had 465), but the *difference* between the plate and the screen.

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

« Last post by bmccowan on June 03, 2026, 12:48:44 pm »

It’s a great sounding amp, but different from the originals that I’ve played. Cleaner and punchier, which I attribute to the Marshally power amp. I think the low screen voltage from the Silvertone voltage doubler contributes to the originals’ signature tone, which is a bit mushier.

the changes to the power supply affect more the "feel" of the amp. I would say the Silvertone has a spongier, compressed feel with less headroom, and the Mojotone version is more punchy and dynamic, loud.

I suspect these comments hit the nail. Overall higher voltage has those effects. If wanting the sound of the original I would seek out a lower power PT. For example Mojo has a lower power JTM45 PT, and Hammond has many options. It'd be interesting to compare the B+ voltages of originals vs the Mojo . 
Further, I see that the difference between plate and screen voltage is dramatic. I didn't know this when I owned one - I was a Freshman in High School 1968! At the time I was looking to trade it for a solid state amp! Who knew - not me.

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

« Last post by bmccowan on June 03, 2026, 12:23:23 pm »

Several years ago I rebuilt a National Sportsman 1201, which is also often identified as a 1202. Valco, like Gibson had no respect for numbers!
Roarshock asked if I had a schematic, as he is thinking of building the circuit. I never found an original but drew one up from my amp. After replacing the caps, the mic channel sounds great. The instrument channel lacks the gain needed to drive the tone stack and is not strong enough to defend itself. I thought I had made a mistake, but searching out other owners I found they all had the same experience. If I wanted to mod it, I would adjust the tone stack or make it a one channel amp cascading the two sides of the 6SL7. I am not likely to mod mine, as I just consider it too cool for school, but maybe some ideas for Roarshock?

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

« Last post by johnfromcyrene on June 03, 2026, 12:14:35 pm »

Also I took a look at the schematic again and to my eye it looks like all they really changed other then deleting the trem and reverb was change those diodes for the doubler circuit.

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Tube Amp Building - Tweaks - Repairs / Re: Bias Help for Rob Robinette JCM800 6V6

« Last post by sluckey on June 03, 2026, 09:48:36 am »

Here's a pic showing a trimmer mounted on the bias balance pot in a Fender Bassman amp. I know it's a slightly different circuit but how you would solder the trimmer to the back of the pot is the same procedure. Of course, the style of your trimmer may require a slightly different mounting method.