Introduction and Franken Fender AA1164D Build

[Slumpin]

Hello Amp Garage Folks,

Long time lurker/tinkerer here. I've done a bunch of builds ranging from great to awful but I've never been much of the sharing type. Might be that the covid isolation is finally getting to me but I'd like to make more of an effort to reach out to people, might as well be people with a common affliction.

I'm building a pair of AA1164ish amps to satisfy the needs of my modest basement recording studio and thought I would document the process and share my results. I've decided to go the route of robrobinnette.com's reverb trem delete. I've added a balanced input for reamping and long cable runs, an effects loop (I know it's rather pointless but I have an extra tube socket and I'm stubborn) and an output attenuator. Also, mid control looks fun. Did I draw that up right? I should double check that. I have a couple of Hammond AO-43 amps lying around so I will use the OT/PT and chassis from them. I will be building two identical amps so I can wrap myself in stereo chorus on lonely winter nights and again, stubborn.

I'm self taught in electronics so I have all kinds of dumb questions. So apologies in advance for that.

Here's what I have so far. I basically copy pasted from Rob's schematic and added a tube effects loop called the Claus-Umbleator I found somewhere, also added a balanced input circuit from JLM audio and an output attenuator I found on guitar.com. This is the first time I've drawn up a schematic and I don't really know what the etiquette is for "borrowing" other peoples designs. Can I even put my name on this? not sure how it works.

Anyways, here's what I have so far, next step is layout. If anyone cares to point out any glaring mistakes I would be eternally grateful.
I will update soon with my progress. Any feedback whatsoever is muchly appreciated.

Thanks!

Studio Slump R0.pdf

Studio Slump R0-page-001.jpg

AA1164_Princeton_Reverb_Delete_Schematic.png

Claus-Umbleator.jpg

[xtian]

Hi, and welcome to TAG.

I don't understand the point of the FX loop. You cannot generate much (if any) distortion from the stages prior to the loop, making the loop pointless.

[TUBEDUDE]

At a glance: missing screen grid resistors on the output tubes. The bias cap is reversed. The inline resistor after the second stage is identified as a .02 cap.

[pdf64]

The 47k cathode resistor on V2a should probably be 47ohms?
The three shorting input jacks on the balanced input seem weird - would an xlr be used?
What’s the purpose of the 100k variable resistor between the balanced and regular input sections?

...I don't understand the point of the FX loop. You cannot generate much (if any) distortion from the stages prior to the loop, making the loop pointless.

And the 250k value of the send level pot will result in mid way settings raising the ‘send’ output impedance to 62k, making the CF stage seemingly kinda pointless and the loop as a whole doubly pointless.

[Slumpin]

Thanks for the replies!

The 47k cathode resistor on V2a should probably be 47ohms?

Yes it should

The three shorting input jacks on the balanced input seem weird - would an xlr be used?

I was planning on using one of those isolated plastic Marshall TRS switching jacks. My train of thought was:

- when nothing is plugged in, balanced input is in the circuit but is grounded so hopefully quiet? I'm trying to avoid having to have a selector switch for the inputs.
- when you plug in the normal input it disconnects the balanced input circuit
- when you plug in the TRS it ungrounds itself. I'm leaving the sleeve floating for the time being, I'll play around with grounding to see what works best.

Is grounding the input before the input transformer necessary? maybe just using an xlr makes more sense. If I want this thing to be quiet when nothing is plugged in will I need to ground the normal input?

Should be easy enough to play around with this stuff after it's built either way. I can always add an input select switch.

What’s the purpose of the 100k variable resistor between the balanced and regular input sections?

From what I gather, the 100K is to vary the impedance. The transformer is a 600:9.6K microphone input transformer wired backwards. Here's a link to the JLM kit I'm going off https://www.jlmaudio.com/shop/repro-pas ... x_prices=1

At a glance: missing screen grid resistors on the output tubes. The bias cap is reversed. The inline resistor after the second stage is identified as a .02 cap.

yep, good catch, thank you much. I'll add some 470s in there too.

[Slumpin]

I don't understand the point of the FX loop. You cannot generate much (if any) distortion from the stages prior to the loop, making the loop pointless.

And the 250k value of the send level pot will result in mid way settings raising the ‘send’ output impedance to 62k, making the CF stage seemingly kinda pointless and the loop as a whole doubly pointless.

Ok, dreams crushed but agreed. My initial thinking was I'd stick my Strymon Flint and a delay where the reverb used to be, but that's dumb, I get it. Actually the whole point off this thing is to avoid having reamp boxes and attentuators and cables everywhere so running an effects loop for the sake of using up a tube socket is just plain irresponsible. Scrap that.

Here's the amended schematic. Basically just a PR now

Studio Slump R1.pdf

[pdf64]

...

What’s the purpose of the 100k variable resistor between the balanced and regular input sections?

From what I gather, the 100K is to vary the impedance. The transformer is a 600:9.6K microphone input transformer wired backwards. Here's a link to the JLM kit I'm going off https://www.jlmaudio.com/shop/repro-pas ... x_prices=1 ...

The input impedance will mostly be set by the 1k track resistance of the balanced level pot. The 100k pot will make negligible difference to anything as it's swept over its range.
Apologies but I couldn't be bothered to work out the schematic from the string of photos linked to in the JML page.

[xtian]

I couldn't be bothered

You're in UK, Pete. Shouldn't you be saying, "couldn't be arsed" ?

[pdf64]

I couldn't be bothered

You're in UK, Pete. Shouldn't you be saying, "couldn't be arsed" ?

So crude!

[Slumpin]

The input impedance will mostly be set by the 1k track resistance of the balanced level pot. The 100k pot will make negligible difference to anything as it's swept over its range.
Apologies but I couldn't be bothered to work out the schematic from the string of photos linked to in the JML page.

Fair enough. Looks like the resister has more of an effect when driving smaller loads, maybe a fuzz pedal or something. So, won't make much difference with the 1M input. One less knob.

[pdf64]

Is being able to adjust the balanced input’s impedance something that would be useful? It’s easy enough to do.

[Slumpin]

I guess I didn't think about this too hard but... Basically I want to be able to run a balanced line level output from my recording desk all the way across the room to the amp. I don't need variable input impedance just a fixed input impedance that is happy to see the lowish (100 - 600Ohm) output impedance of my recording gear, a level control would be useful. With the level pot wide open the 600:9.6K seems like a good match. Is there another way of approaching this?

[pdf64]

...I want to be able to run a balanced line level output from my recording desk all the way across the room to the amp. I don't need variable input impedance just a fixed input impedance that is happy to see the lowish (100 - 600Ohm) output impedance of my recording gear...

A high impedance input is absolutely delighted to see a signal that has a low source impedance, such as your desk’s output. It’s referred to as an impedance bridging connection, which since the 70s has been the basis of most audio systems. It results in minimal loss of signal voltage between source and input.
Impedance matching is only used in RF and a few specific applications.
I’m doubtful that it would be beneficial (in regard of audio fidelity) to feed a desk output (ie +4dB pro line level) into a reversed mic transformer; the magnetic circuit is designed for mic level, not line, and to step that line level up probably overdrive the tube input stage (or is that the point?).
Rather just connect one side of the balanced line to a 10k pot (to control the level), then to the input.

[Slumpin]

hmmpf. I guess I've been using re-amp boxes my whole life not really thinking about what they do. Makes sense, low output impedance, high input impedance, everyone is happy. My main concern is noise from a long cable run. I tried running an unbalanced guitar level signal with a buffer to my 5E3 from 30ft away and it introduced a huge amount of hum, I thought using a balanced signal would eliminate that issue. If I tap one leg of the balanced signal do I just end up with another 30ft antenna?

[pdf64]

Seems weird - unless being run near some serious power distribution transformers, I expect a decent buffer to be able to feed a 30’ unbalanced cable without any hum being picked up. Not much point in a buffer if it couldn’t fix that sort of thing.
Whatever, it should be simple to knock together a cable between your desk and an amp to check whether a balanced line and coupling is necessary.