Metropoulos Forum

I recently bought a neat little 12AU7 powered, .25 watt combo amp. The amp is made in the gutted chassis of an old Marshall/Park G10 MKii. It's a cool little amp and really delivers a nice plexi-style tone in a very small package. I'm not the builder, as the following question will confirm:

I'd love to add a MetroAmp zero loss FX loop to the amp, but I could use a little help. I don't have the schematic for the amp, and am hoping someone here can help me identify B+ from the following pictures:

http://imgur.com/a/kELWz" onclick="window.open(this.href);return false;

Hopefully this is a simple request and the pictures are detailed enough. If not and better pictures are required, please tell me and I'll take a few more.

Sorry for the obvious newbie questions. Thanks for whatever help anyone can provide!

Bump -- any help at all would be appreciated! I think if someone can point out the B+ connection to me I can figure out the rest.

So I reached out to the person who built my amp, and he had the following to offer w/r/t my question:

For the 12au7 output tube, pin 1 gets approx 130-135v via the blue wire on the output transformer. Transformer red lead is high voltage from power supply.

The gold wire is approx 120v and goes to 3 100k resistors.

No schematic online. My own design.

Based on the above info, can anyone tell me if B+ on this little quarter-watt amp putting out enough power to drive this FX board?? Is there a formula I could use to determine the correct decoupling resistor value, as it looks like it's going to be a lot less then what's in the instructions -- more like a single or double digit value rather than a value in the "k" range.

Again, I'd appreciate any info anyone could pass my way. Thanks.

OK, the silence is deafening, but maybe I can simplify my request a bit:

How much power does the zero loss FX board need to operate? I don't know if my little 12AU7 powered amp has a strong enough B+ to power it, but I might be able to figure something out if I can get just a little help here. Thanks.

Most are going to be very hesitant to chime in on a one of a kind amp with no known schematic. Pics provided give a general idea, but not quite enough detail to see everything well enough. People don't want to mistakenly mislead you, including myself, if they really have no real way to know if the circuit is a proper fit or not.

The B+ details you added last night do help though, and the B+ is indeed much lower than the ZL loop was designed for. While there's not quite enough info to say how the loop would work in there based on the circuit topology, the very low B+ would definitely rule the ZL loop out for this amp. Sorry about that.

SDM wrote:Most are going to be very hesitant to chime in on a one of a kind amp with no known schematic. Pics provided give a general idea, but not quite enough detail to see everything well enough. People don't want to mistakenly mislead you, including myself, if they really have no real way to know if the circuit is a proper fit or not.

The B+ details you added last night do help though, and the B+ is indeed much lower than the ZL loop was designed for. While there's not quite enough info to say how the loop would work in there based on the circuit topology, the very low B+ would definitely rule the ZL loop out for this amp. Sorry about that.

Thanks. Not the answer I was hoping for, obviously, but it does seem like the right assessment. Thanks very much for taking the time (and going out on a limb) to answer my question.

Out of curiosity, do you know how much power the ZL board does require to operate properly?

Thanks again!

I believe 285 to 315v is ideal?

Not sure how low it can go?

neikeel wrote:I believe 285 to 315v is ideal?

Not sure how low it can go?

Yeah that is roughly the typical/normal voltage range to have the loop board run at, but it was designed to work just fine even lower too (mainly due to the Metro kits with low power options around at the time). So, can extend the ideal range down to around 250V.

It can go lower yet, some have used it with close to 200V reaching/powering the loop board, but you'll likely starting to sacrifice some headroom at that point. Around 170-180V on the loop board is when voltages are just too low, beyond the limits it was designed for and it will not function properly anymore.

So, ultimately, could consider around 250V the "ideal" low end to use the loop with. Around 210V to 250V "acceptable" in some cases (depends on signal swings and such) but may be limiting headroom. Between 210V and 180V a very "grey" area, not recommended, but some odd cases may just get away with it. Below that absolutely no go, loop won''t be operating near what it was designed for, will effectively be shut down/off

All voltages listed here are at the loop B+ input by the way, always want to have a decoupling resistor in use, so B+ tapping point in the amp would need to be slightly above these numbers at the least.