AMT SS-11A Tube Preamp with AMZ Power Supply

[spoontex]

Hello,

I've build an ampless pedal board. Consist of these pedals.

Comp, AMT SS-11A, delay, Torpedo CAB M.

I need some specific voltages, so I decided to modify my AMZ power supply.

http://www.muzique.com/tech/powr6.htm

- I've changed one TO220 LM7809 for an LM7812. For the Torpedo Cab power requeriments ( 12v 250mA ).

- For the AMT SS-11A, I've changed the LM317 for a LM388T, that can draw more current, and adjusted the voltage divider for 12v. AMT needs 12v 0,5A. Tube heaters in series??

So, all works. But in case of AMT the voltage down to 11,25V... This is normal? I need to adjust the voltage divider for 12,6V?

With the original AMT power supply, the voltage is more estable... 11,80V

Thanks.

[Marcos - Munky]

AMT needs 12v 0,5A. Tube heaters in series??

It haves two 12AX7. Each one can be wired with heaters in parallel (300mA @ 6.3V) or in series (150mA @ 12.6V). So, we have three options:
1-each tube with heaters in parallel, then tube 1 in series with tube 2. You'll get 300mA @ 12.6V.
2-each tube with heaters in series, then tube 1 in parallel with tube 2. Same thing, 300mA @ 12.6V.
3-both tubes with heaters in parallel. That makes 600mA @ 6.3V. While the voltage isn't an issue (it can easily be dropped), the current doesn't match the requirements. I just listed this one because it is an option.

So we can't be sure on what's happening, it could be 1 or 2, but what we can be sure is something is in parallel and something is in series.

But in case of AMT the voltage down to 11,25V... This is normal? I need to adjust the voltage divider for 12,6V?

The voltage is 10.7% below the ideal voltage for the heaters. While the best idea is to keep the heater voltage as close as possible to the ideal voltage, there's a tolerance on this voltage. Some people say it's best to stay under 10%, some people says 15%. And some people build Valvecasters running @ 9V, so the heaters will have a difference of 33% no matter how you wire the heaters. I'd say you're fine, but you can adjust the voltage if you want.

[spoontex]

I use 6n2p tubes. So 6,3V in series is 12.6V and 340mA. Right?

[Marcos - Munky]

Yep, you're correct. 2 6N2P wired in series makes about 340mA @ 12V.

But the original uses 12AX7, right? It is modded to fit the 6N2P or you just swapped the tubes?

It's a good idea to open your pedal and check how the heaters are wired. The 12AX7 and the 6N2P have different heater wirings, and maybe you're feeding the 6N2P heaters with a way wrong voltage.

I mean, 12AX7 heaters are on pins 4, 5 and 9 with 4 and 5 for heaters in series (12.6V) and 4+5 and 9 for heaters in parallel. 6N2P just have one choice for heater wiring, that's pins 4 and 5 for heaters in parallel (6.3V). If your pedal is working with the 6N2P, then you're getting voltages on pins 4 and 5.

So, if the original uses 12AX7 and yours isn't modded to fit 6N2P, we can narrow down the heater wiring to option 2 I said before (each tube with heaters in series, then tube 1 in parallel with tube 2). It coudn't be option 1 (each tube with heaters in parallel, then tube 1 in series with tube 2), because with the wiring for heaters in parallel for a 12AX7 woudn't make a 6N2P work.

What may be happening then: the original setup expect a 12V heater in parallel with a 12V heater, both fed by 12V and that's fine. But, by just swapping the 12AX7 for the 6N2P and not changing the heater wirings, you have a 6V heater in parallel with a 6V heater, both fed by 12V! Double of the voltage . While it'll probably work, it'll work for some time, because the lifespan of your 6N2P is being shorted a lot.

Remember I'm just assuming this based on the info I got and assuming the original uses 12AX7 and yours is not modded. You have to open it to be sure.

Question: is the glow from the heaters too bright?

[antonis]

- For the AMT SS-11A, I've changed the LM317 for a LM388T, that can draw more current, and adjusted the voltage divider for 12v. AMT needs 12v 0,5A....
But in case of AMT the voltage down to 11,25V... This is normal?

If it's  LM338T adjustment issue, what Marcos said..
If it isn't, something very bad is happening..
(meaning that, in case of a 5A regulator set for 12V with no load goes down to 11.25V when loaded, check load immediately..)

[spoontex]

Yep, you're correct. 2 6N2P wired in series makes about 340mA @ 12V.

But the original uses 12AX7, right? It is modded to fit the 6N2P or you just swapped the tubes?

It's a good idea to open your pedal and check how the heaters are wired. The 12AX7 and the 6N2P have different heater wirings, and maybe you're feeding the 6N2P heaters with a way wrong voltage.

I mean, 12AX7 heaters are on pins 4, 5 and 9 with 4 and 5 for heaters in series (12.6V) and 4+5 and 9 for heaters in parallel. 6N2P just have one choice for heater wiring, that's pins 4 and 5 for heaters in parallel (6.3V). If your pedal is working with the 6N2P, then you're getting voltages on pins 4 and 5.

So, if the original uses 12AX7 and yours isn't modded to fit 6N2P, we can narrow down the heater wiring to option 2 I said before (each tube with heaters in series, then tube 1 in parallel with tube 2). It coudn't be option 1 (each tube with heaters in parallel, then tube 1 in series with tube 2), because with the wiring for heaters in parallel for a 12AX7 woudn't make a 6N2P work.

What may be happening then: the original setup expect a 12V heater in parallel with a 12V heater, both fed by 12V and that's fine. But, by just swapping the 12AX7 for the 6N2P and not changing the heater wirings, you have a 6V heater in parallel with a 6V heater, both fed by 12V! Double of the voltage . While it'll probably work, it'll work for some time, because the lifespan of your 6N2P is being shorted a lot.

Remember I'm just assuming this based on the info I got and assuming the original uses 12AX7 and yours is not modded. You have to open it to be sure.

Question: is the glow from the heaters too bright?

The AMT SS-11b have internals jumpers for set between 12ax7 and 6n2p.


http://medias.audiofanzine.com/images/normal/amt-electronics-ss-11-guitar-preamp-216548.jpg

[spoontex]

- For the AMT SS-11A, I've changed the LM317 for a LM388T, that can draw more current, and adjusted the voltage divider for 12v. AMT needs 12v 0,5A....
But in case of AMT the voltage down to 11,25V... This is normal?

If it's  LM338T adjustment issue, what Marcos said..
If it isn't, something very bad is happening..
(meaning that, in case of a 5A regulator set for 12V with no load goes down to 11.25V when loaded, check load immediately..)

With the original power supply, works great. The voltage only down to 11,80 - 11,90v.

I think something is happening with the voltage regulator.

[Marcos - Munky]

Then, with the heaters adjusted for 6N2P, you're good to go. Don't worry on this voltage drop.