octal & el84 phonograph amp conversion

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yello
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octal & el84 phonograph amp conversion

Post by yello »

So this isn't an 18w - but it might eventually become one. Tube layout is currently:

6SC7
12ax7
dual el84
5Y3

Before I gut and convert, I'd like to get it up and running to hear it with an octal preamp and the current volume/tone arrangement. I've never messed with an octal preamp and this is a good chance to learn.

Here is the original factory schematic, and the parts list along with voltages:

Image
Image

The amp came to me modified from its original state somewhat, these are the things I've noticed which I've tried to capture in a modified schematic:

1) Phonograph and Aux inputs and switching removed
2) Aux power plug and switch and death cap removed
3) 3-prong power added, along with a 3A slow blow fuse
4) 1/4" input jack added - input connected to the .02 cap that feeds the bass pot
5) individual 8 and 16 ohm speaker jacks

Image

The amp turns on and passes signal. Controls work, just not much volume, I assume because it looks like the 6SC7 is bypassed so it is just a power amp. Does not have noticeable hum or noise, but it may be not pronounced given the 6SC7 is bypassed. I was told the amp was brought up on a variac and the filter caps reformed.

What I would like to do is:

Step 1: Get the input corrected to go through the 6SC7, and connect the bass cap to the right point. It looks like the original phono input was removed, and the hanging grid leak cap C1 just grounded. Then a 1/4" input jack connected to C5. Also, adjust components on 6SC7 to produce a good guitar sound.

Possible step 2: If grid leak bias 6SC7 (70 mu single triode right?) sounds anemic - convert it to cathode bias with new components and wiring, or consider going to a different octal like a 6SQ7 (dual triode 100 mu).

Possible step 3: Adjust current tone and volume control components and wiring if needed, and if that doesn't produce a good sound, V/T redesign.

Possible step 4: Consider going to a 12ax7 v1, or ef86 - but keeping rest of amp the same.

Possible step 5: gut and build an 18w variant.

I know I might end up at step 5 ultimately, but I'd love to work through the process so I can learn more about wiring octal preamps.

Does anyone have any advice for step 1, how to properly connect the input so the stock amp works properly to at least hear it before making further decisions? Do I need to connect C1 to the input jack tip, and C5 goes to pin 5 of the 6SC7?

And how to then modify that 6SC7 wiring for better guitar sound?
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yello
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Re: octal & el84 phonograph amp conversion

Post by yello »

I ended up completing step 1 as laid out above, and testing, and the amp sounds great! Little to no noise, and great crunch when dimed.

Not a ton of gain on tap, but a nice range of tone control. It’s a keeper, even if I do more mods to it later.
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zaphod_phil
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Re: octal & el84 phonograph amp conversion

Post by zaphod_phil »

That looks very promising. the 5Y3 rectifier will probably make it quite saggy


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Nature abhors a clean tube amp

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yello
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Re: octal & el84 phonograph amp conversion

Post by yello »

zaphod_phil wrote:
Sat 09/10/22 7:27 pm
That looks very promising. the 5Y3 rectifier will probably make it quite saggy
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Indeed, saggy.

I'm gonna try it next without the negative feedback to hear what that sounds like.
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