Advice needed...

[The4thWatcher13]

Hello, my name is mark and I've been fixing audio equipment of all kinds for a few years now but I'm just getting into amp building and mods. My first foray into this world came from the desire to convert a small PP record player amp into a guitar amp. Needless to say this was a "Widow maker", hence the need for the iso-trans.There were no docs for the amp so I traced it and made a schematic. Added an isolation trans, fuse and started reading a number of recommended books on amp design. My little record player amp is currently working and has one 12AX7 that looks like half is doing amplification and getting feedback for phase inversion for the two 50C5 in the output.
It passes guitar signals fine but it's completely clean all the way up, "moderately" loud and I think can be pushed a little bit more. So here is the part where I need advice. There is so much to consider when "designing" even a preamp stage that my head is ready to fly off! And I feel like I'll never actually get around to doing anything if I don't do Something/Anything! I've attached the "TinyCAD" drawn schematic of the amp for reference but what I find myself wondering is where do I start?! What do I consider first? I've read others say "steal a pre section from a previous build, like a Fender". I've looked at a LOT of schematics but the closest thing I've found to what I think will help is the "Minimalist 18 watt", drawn by Mark Huss, and posts in the 18 watt community here, and I thought this might be a good starting place. I bought a nice set of tube bread boarding sockets from "Trenton Blizzard", I've got lots of parts and know what I'm doing working on and with electronics. Designing things, ...not so much.
So any input before I start destroying stuff would be much appreciated.
I've also added the current voltages taken from this amp and the schematic for reference.
I'm already headlong down the rabbit hole so there's no turning back and some advice at this point I think would do me a lot of good.
Thanks much for reading this, I'm really anxious to hear what you folks have to say.
Mark

The iso-trans is a Triad N-68X. I've reversed the primary and secondary because the voltages from a 120 AC wall supply were too high (approx 135/140) at the multi cap (40/40/40 -150V) (Maybe with the addition of another 12AX7 and another electolytic the iso trans can/should be reversed again?)

Currently voltages are:
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) +0.001V

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

[TriodeLuvr]

That's an interesting circuit. I didn't realize the inexpensive AC/DC phonos used cathode feedback. Probably done because the secondary of the output transformer couldn't be connected back to the circuit through a feedback network without creating a shock hazard.

Yes, you can add one more 12AX7 in series. There are so many variations on preamps, it's difficult to offer advice. In addition to the preamps here, there are examples at the AX84 site. If you're satisfied with the sound of the existing amp and the tone control, the addition could be as simple as two stages in cascade. Actually, you might find that one stage is enough, depending on other factors.

Don't drive yourself nuts regarding filament voltage. It's better to have it on the low side than to apply too much. I have one commercial hi-fi preamp circuit that intentionally heats its EF86 (6.3V filament) from a 5.7 VAC winding. Filament voltages are generally spec'd loosely.

Good luck with the project!

Jack

[JMPGuitars]

Don't drive yourself nuts regarding filament voltage. It's better to have it on the low side than to apply too much. I have one commercial hi-fi preamp circuit that intentionally heats its EF86 (6.3V filament) from a 5.7 VAC winding. Filament voltages are generally spec'd loosely.

Low heater voltage can be worse than high heater voltage. But many tube datasheets include info for what is an acceptable variance in heater voltage. Most I've seen are +/- 5 to 10% of their typical use rating.

[The4thWatcher13]

That's an interesting circuit.....
Yes, you can add one more 12AX7 in series.....Actually, you might find that one stage is enough, depending on other factors.
Don't drive yourself nuts regarding filament voltage. It's better to have it on the low side than to apply too much. Good luck with the project!
Jack

Hi, jACK, and thanks for the info and encouragement. Whether I'd need one or both halves of an additional 12AX7 was one of the questions I've been pondering as well as the over all sum of heater voltage. I'm glad I got the bread board so changing things around will be that much easier. There are so many different things to consider I think what I'm having trouble with has been what to start with (the beginning?!)..regardless of the topology decided upon
So I'm going to start with putting the heater in series with the rest and see what I get, then move on from there.
Thanks again Jack, more to follow...

[The4thWatcher13]

Low heater voltage can be worse than high heater voltage. But many tube datasheets include info for what is an acceptable variance in heater voltage. Most I've seen are +/- 5 to 10% of their typical use rating.

Hey JMPG, thanks for the input. As I've been concentrating on fixing not building all this time and I'm reverse engineering my brain over here,..it hurts. But I can catch a clue; start with the heater and move forward. Thanks again, more to follow...
Mark

[TriodeLuvr]

As an afterthought, I believe you'll find one stage of the 12AX7 will be enough. That amp was probably designed for a ceramic cartridge with an output of one or two volts. A guitar signal needs no more than about X50 voltage gain to get to the same level. My suggestion is to add one stage of amplification and evaluate. A second stage in cascade, with all the attendant issues of layout and stability, might be exponentially more difficult. Fun project!

Jack

[JMPGuitars]

If you want to learn more about general preamp design for guitar amps, Merlin Blencowe (ValveWizard) has a book dedicated to it. He's also got a lot of information on his website. http://www.valvewizard.co.uk/index.html

Thanks,
Josh

[zaphod_phil]

Merlin's web site has some great advice. However, I would warn against taking everything he says as being gospel. Merlin sometimes makes a big deal of some trivial things, which I really don't think make any noticeable difference.

[The4thWatcher13]

As an afterthought, I believe you'll find one stage of the 12AX7 will be enough..... A second stage in cascade, with all the attendant issues of layout and stability, might be exponentially more difficult. Fun project! Jack

Fun indeed, it's been a real eye opener. Thanks again for the info. The simpler the design the better right now.

[The4thWatcher13]

If you want to learn more about general preamp design for guitar amps, Merlin Blencowe (ValveWizard) has a book dedicated to it. He's also got a lot of information on his website. http://www.valvewizard.co.uk/index.html Thanks, Josh

Josh, that was the first book I bought when I considered doing this project. And I've been deep into his website as well as Rob Robinette's site. It seems there is a lot of good info there too. Merlin's book is rather "granular" and that's what has my head about to explode. Generally I like to understand the things I read before I progress through a book but I think in this case I'll continue with the ol' college method. it all the way through, understood or not, and then when I have a question or issue go back to where he covers the issue. It leaves you with that "up in the air" feeling but I'm up in the air already so.. Thanks Josh, I appreciate your input.

[The4thWatcher13]

Merlin's web site has some great advice. However, I would warn against taking everything he says as being gospel. Merlin sometimes makes a big deal of some trivial things, which I really don't think make any noticeable difference.

Thanks for the input, Phil. I get what you're saying. The guy is definitely a Scientist! And I can't help getting the feeling that everybody that builds or mods amps isn't doing it with his years of experience or technical knowledge, but achieves success none the less. And that's encouraging. Thanks again, Phil.

[The4thWatcher13]

Insight? I keep getting the feeling that one thing I'm trying to find is "Process". That is; what people consider first, secondly and so forth. After being pummeled by Merlin's books' first chapters I started skipping around a bit and I think I saw reference to this (Process) in later chapters so I'll keep reading until I hit the end. But I can't help feeling that any one that "knew how to do" what I'm trying to do would go about it, or consider things, in a particular order. Undoubtedly most would have their own personal order but an order none the less. I'm generally an "understand by doing" kind of guy but I like to educate myself as much as possible so my undertakings are as sound as possible at the onset. This, I think, is what's been bothering me as of late. The need for a bit of grounding. (..pun intended!)

[TriodeLuvr]

Yes... process. That takes precedence over the nuances of circuit design, doesn't it? In the end, they're related, but it's like the outline of a novel vs. the sentences.

In your situation, I would consider the power supply first. AC input voltage to the amplifier is caught between two limits. On the high side, the filter caps are rated for 150V and the 50C5s are specified at +135V max on the anodes. That's anode-cathode voltage, and it will be equal to B+ minus the drop across the output transformer cathode windings, anode windings and the common cathode resistor, R11. I would guess the actual voltage impressed across the tubes will be about 10V to 15V less than B+.

Also, when considering the upper limit, we have to think about cold starts. When power is first applied to the amp, the tubes are cold and draw no current from the B+ supply. This means the filter caps will temporarily charge up to the peak value of the AC. 115VAC input has a peak value of about 160V. Clearly, the filter caps are operating in their "surge" value rating at turn-on. This implies the amplifier should not be powered with anything greater than 115VAC, and I would consider this value to be a hard ceiling.

On the low end of appropriate voltage, there's the voltage required for the filament string. The amplifier was obviously designed for 115VAC nominal, as evidenced by the sum of the series filaments (112.6V). You have some leeway here, but the addition of another 12AX7 (or maybe a 12AV6, which is one section of a 12AX7) really throws this out of whack. You need about 125VAC for the filaments in that case (design center). That's clearly too much for the filter caps, and this leads to a problematic conclusion: there is no single-voltage AC input that can satisfy the requirements of both the B+ and the filaments if another tube is added to the string.

As with most things, there are several ways around the problem. To my mind, the simplest would be to add a 6.3VAC/3A filament transformer and replace the 50C5s with 6CU5s. The 6CU5 has an identical pinout and functionality, other than filament voltage. You'll also need to rewire the 12AX7 for 6V operation. There are several advantages in this, including reduced filament hum leakage in the low level stages. This will become more important when you add the additional stage of amplification. In short, adding a filament transformer will give you a much better foundation for the other modifications necessary to run a guitar through this amp.

Jack

[The4thWatcher13]

Jack, thank you so much for your guidance. I appreciate you taking the time to reference my schematic and voltages and laying it out in a way that I can understand. I'm totally picking up what you're laying down, bro. I know it sounds funny but somehow I understand what you're saying. I've been storing up a lot of tubes and iron for years so I may even have a suitable filament transformer around here as well as some 12AV6's & 6CU5's, so your suggestions are a definite possibility. I realize also that I'm sort of swinging for the fences at first up at bat here, but it's the journey and greater understanding I'm more interested in at this point , not having the prettiest amp on the block. So, I'm off to my storage space to see what I've got on hand. I really appreciate your help Jack, I'm taking notes and doing homework!

** I'm definitely getting the "Wizard" vibe from Jack's kind guidance but any and all input and comments are welcome and appreciated!
More to follow...
M

[TriodeLuvr]

If you change to 6.3V filaments, the preamp tube would need to be a 6AV6, not the 12AV6 (assuming you don't use another 12AX7 instead). The reason for this is that the 12AV6 does not have a center-tapped filament like the dual triodes. It won't operate on a 6V filament supply.

Jack

[The4thWatcher13]

If you change to 6.3V filaments, the preamp tube would need to be a 6AV6, not the 12AV6 (assuming you don't use another 12AX7 instead). The reason for this is that the 12AV6 does not have a center-tapped filament like the dual triodes. It won't operate on a 6V filament supply. Jack

Thanks Jack, I've come up short for a filament trans so far but I'm still looking. Same for tubes but there's a good chance I've got em. I Just need to keep looking. I found the right trans online so that may be the way to go, it's cheap, but only after I exhaust my search. I did find a 6.3-6A and a 6.3 - 0.600A ..of course.

[JMPGuitars]

Thanks Jack, I've come up short for a filament trans so far but I'm still looking. Same for tubes but there's a good chance I've got em. I Just need to keep looking. I found the right trans online so that may be the way to go, it's cheap, but only after I exhaust my search. I did find a 6.3-6A and a 6.3 - 0.600A ..of course.

What are you looking for exactly?

[TriodeLuvr]

The filament transformer doesn't need to be exact (amperage), but 6A might be too much. The transformers are rated for the voltage they deliver at full load. Running a 6A transformer at ~3A will probably over-voltage the filaments, especially if it's an older transformer intended for 115/117V operation. A 3A transformer would be perfect.

6CU5s are pretty cheap on eBay. I checked on that before suggesting them as replacements.

Jack

[The4thWatcher13]

What are you looking for exactly?

The 6.3V - 3A filament transformer. I thought I might have one in storage. But there's one on mouser for $13 bucks. Can't beat it, if you can't find it!

[The4thWatcher13]

The filament transformer doesn't need to be exact (amperage), but 6A might be too much. The transformers are rated for the voltage they deliver at full load. Running a 6A transformer at ~3A will probably over-voltage the filaments, especially if it's an older transformer intended for 115/117V operation. A 3A transformer would be perfect.
6CU5s are pretty cheap on eBay. I checked on that before suggesting them as replacements. Jack

That may be my "go to" if I can't find any around here. I've got lots of tubes. I was well on my way to becoming a tube hoarder! But I've got it under control, ...sort of. Problem is the vast majority are un-organized. Doh!