Record Player Amp to Guitar Amp Input Needed

[martin manning]

Swap the iso transformer’s primary and secondary. 120 VAC input will produce ~115 VAC out.

[AAVA]

Swap the iso transformer’s primary and secondary. 120 VAC input will produce ~115 VAC out.

You know, I thought that there might be a vintage solution but didn't think that the solution would go back as far as "the old switch-a-roo"!
I suppose the "secondary" having the shielding wouldn't matter? And that it should stay connected to the chassis?
Thanks Martin

[martin manning]

This transformer is Triad N-68X? It's supposed to be 1:1, but reversing it might work for you. It'll be interesting to see. The shield should work either way as it's between the primary and secondary.

[AAVA]

This transformer is Triad N-68X? It's supposed to be 1:1,
but reversing it might work for you. It'll be interesting to see. The shield should work either way as it's between the primary and secondary.

The iso-trans is indeed the N-68X. I've reversed the primary and secondary and the voltages were more surprising than the original measurements.
AC:
120 VAC to primary = 98.1 VAC on secondary
Heater voltages:
V3(50C5) pin 4: 93.7 VAC - pin 3: 51.4 VAC
V2(50C5) pin 3: 51.4 VAC - pin 4: 11.7 VAC
V1(12AX7) pin 4: 11.7 VAC - pin 5: 0.040 VAC

DC voltages:
C3 Multi-cap: C3-A = +118 V - C3-B = +116.5 V - C3-C = + 112 V
V1: pin 1(plate) +63.5 V - pin 6 (plate) +65.2 V - pin 8/3 (shared cathode) +0.505 V - pin 2 (grid 1) -0.001 V - pin 7 (grid 2) +0.001 V
V2: pin 7 (plate) +111.1 V - pin 6 (screen) +116.7 V - pin 2 (grid) +0.025 V - pin 1 (cathode/supp grid) +9.0 V
V3: pin 7 (plate) +110.4 V - pin 6 (screen) +116.5 V - pin 2 (grid) +0.028 V - pin 1 (cathode/supp grid) +9.0 V

Looking at the spec sheets for these tubes it appears to me that we're in the ball park now. Supply cap voltage is definitely making me feel better about standing where I am. So I plugged the guitar in and low and behold it sounds great. Nice and clear at max volume, which could probably improve, and has very little hum audible at zero signal! The previous statements that "Stevem" and "mhuss" made about adding another 12AX7 certainly make more sense now that this thing is safe to operate, measure voltages and play through. Therefore a little (more) thought is in order. I have the tubes, sockets etc needed to modify this further but since I just started the "Designing Tube Preamps for Guitar and Bass" buy Merlin Blencowe, I'm not quite ready to design the mods but I believe I can fathom any recommendations those here are kind enough to submit. If it's not quite time for that yet I'm good with that too and stand ready for comments on what I've currently got going. Thanks again to all that have helped me get this far. I'm now certifiably hooked! And now...dinner!

[thetragichero]

when in doubt i look at the tried-and-true fender 100k plate/1k5 cathode bypassed by a capacitor (22uf? lower if you want less low end). volume control, some sort of tone control(s). look at schematics for amps you like or wanna try and *ahem* borrow what worked for somebody else. fortunately even a small supply of resistors and capacitors means you're not locked into specifics of you get the itch to tweak

[AAVA]

when in doubt i look at the tried-and-true fender 100k plate/1k5 cathode bypassed by a capacitor (22uf? lower if you want less low end). volume control, some sort of tone control(s). look at schematics for amps you like or wanna try and *ahem* borrow what worked for somebody else. fortunately even a small supply of resistors and capacitors means you're not locked into specifics of you get the itch to tweak

That's certainly one thing I'm doing right now. Sites like schematic heaven are a true treasure trove of small and large amp schematics.

[AAVA]

Howdy folks, I'm back! I've had my nose buried in tube books and websites since my last post and trying to apply what I've learned to what I've got going on here with this record player to guitar amp conversion. Instead of asking a million and one questions I thought I'd just ask folks what they'd consider doing if this was their project. To refresh the group memory a little bit; it's a safe, working amp currently, passing audio and amplifying at around a 10:1 ratio, wide open. [0.5 mV input at 1K = approx 5.0 V at output.] Working voltages are posted just a few posts back in this thread as well as the updated (Iso-Trans) schematic. And I'd be happy to provide any other measurements necessary.
The statement was made earlier on in the thread that I could increase the gain/output with the addition of another 12AX7 tube. It is with this end in mind that I've been mashing tube learnin' so seriously these past couple of weeks. So any thoughts about adding another pre amp tube or, in reality, ANYTHING that has anything to do with this project is welcome.
My goal is to try to understand why you would do what you'd do, as it applies to this amp. You can certainly tell me why and then, hopefully, I'll be able to get my head around it and ask reasonable questions. So, kind friends, what would you do with this? What do you see when you look at the schematic? And so forth. I'd really appreciate knowing. Then we can get into the thick of things.
Thanks again,
M

PS. I got some super cool bread board tube sockets from "Trenton Blizzard" online. Nice assemblies for bread boarding tube circuits. They've got sockets for at least 5 different pin number tubes,... and I'm itching to try 'em out!

[thetragichero]

personally I'd rewire the preamp to something more traditional and add a phase inverter with the second little bottle. 100k anode/1k5 22uf cathode is the standard fender stage. remove the volume control from the input and replace with a 1M resistor, go gain stage->volume (and maybe tone)->gain stage->(tone control if not added earlier)->phase inverter

[AAVA]

..personally I'd rewire the preamp to something more traditional and add a phase inverter with the second little bottle. 100k anode/1k5 22uf cathode is the standard fender stage. remove the volume control from the input and replace with a 1M resistor, go gain stage->volume (and maybe tone)->gain stage->(tone control if not added earlier)->phase inverter

Hey "Hero", thanks for the input. It made me think about a few things. So I'm gonna take a few more measurements and be back with a questions or two.

[AAVA]

..personally I'd rewire the preamp to something more traditional and add a phase inverter with the second little bottle. 100k anode/1k5 22uf cathode is the standard fender stage. remove the volume control from the input and replace with a 1M resistor, go gain stage->volume (and maybe tone)->gain stage->(tone control if not added earlier)->phase inverter

Back again, after getting "Hero's" recent response and continuing to read a LOT the impression has been made that most, if not all, amp designers can only do so much "on paper". But once you've got a a good idea of what you're going for all you can tweak both the electrical characteristics and audio characteristics to your preferences. In regard to "Hero's" most recent post(s) my plan is to keep the single volume and tone control that currently exist, though connections to the volume pot will certainly be modified. I've also been researching the info "slucky"' offered on page 2 of this thread about Vox's tone cut circuit, which I think I'll leave as is to start. "Hero" also mentioned about "adding a phase inverter with the second (existing) 12AX7". I've been looking at what the amp is currently doing with audio and have found that the existing 12AX7 is already working as a phase inverter for the push pull output section. (see images) So I'm assuming that all I may have to do is add gain via another 12AX7 before the existing volume control and phase inversion stages, i.e. gain stage-->volume-->gain stage-->phase inverter--> tone control-->output.
Since this amp is so "simple" compared to the multitude of designs I've been looking at I'm constantly on guard to not over complicate things; if that's even possible. Even thought I've been trying to use, in general, older simpler amp designs. Vox, Ampeg, Fender, Garnet etc.
So, if I'm right and the existing AX7 can be left "the way it is" for now, then it's time to start drawing out the necessary modifications to add the other gain stage(s) and contemplate plate voltage for the new 12AX7.
Below I've included some shots of a scope screen illustrating what I've got going right now...
Input is a 1K sign wave at 1VAC: Volume at full / tone at midpoint unless otherwise noted.

Pic 1: Plate outputs of the existing 12AX7: V1-A/B (pin 1=A / pin 6=B on scope) V1-B inverting signal from V1-A 180 deg. out of phase signal for grid of V3.
Pic 2: Grids of V2 / V3 (V2=A / V3=B on scope) Volume at full / Tone at max.
Pic 3: Grids of V2 / V3 (V2=A / V3=B on scope) Volume at full / Tone at mid.
Pic 4: Grids of V2 / V3 (V2=A / V3=B on scope) Volume at full / Tone at lowest.
The signal at max volume and mid tone produces the least sign wave clipping of (V2), which is what I guess I'd expect from a record sound reproduction bent, but not exactly what I'd want for this amp I think, and will most likely need to be dealt with later, possibly along with V2 vs V3 gain factors. (..another assumption)
I'm wondering if I tweak the feedback network of R6, R7, R8 a little bit I might be able to get a little more symmetrical clipping and increased gain from V3.
Also, looking at the plate voltages at max, I'm wondering if the current (original) multi cap is lacking or if the output imbalance is due to the feedback network?
I think I need to set me meter to AC/DC to see both components at once. I thought I had done that but it doesn't look like it.
Anyway, I'm getting ahead of myself,..all opinions are welcome, I'm going to reign myself back in and concentrate on adding another X7 to the front end.
I haven't really studied power supplies (proper) yet but I've been wondering how to power the X7 that's going to be added.
Q: ..add another electrolytic (47uF / 150V)? Daisy chain from existing 'X7 plates? ..I have NO IDEA! This is fun!
So I guess it's time to peruse the power supply issues as well. Thanks again to all for your input, I'm heading back to the books for a few,
M>