PA Head to 5E3, Single Input, Volume, Tone

[F&A Electric]

Hello all, this is my first post on this forum. I have searched here many times and find the posters here to be extremely knowledgeable and helpful. Thank you in advance!
I’ve restored 4 old PA heads, and have a foundational understanding of amplifier circuits.
My current project is building a 5E3 circuit in a Muzak 1008 PA head, and I would like to maintain an original look to the amp.
This means I’m going with a single input, volume and tone controls. I have rewired the V2 9 pin socket for 12AX7 from a 7199.
Interactive 5E3 volume controls don’t interest me, and I understand the compromises with ditching the low inputs and normal channel.

I have read some older posts detailing stagger tuning a parallel triode from Bruce at Mission Amps, and want to try something similar.

I have 33k grid stop resistors to each V1 grid, shared 620 ohm cathode resistor, 25uf/25v bypass cap, and two individual 100k plate load resistors.
For coupling caps I want to use different values, a .1uf and a .033uf, and run these to my volume pot.

So here is my question, should I land my two coupling caps on the middle lug of the volume control or do I need mixing resistors for best results?

[loudthud]

I'm a little confused. Are you using both triodes from the same input jack? If so, then you need a switch to select only one output to the Volume. With both outputs connected to the Volume, the second triode will make very little difference.

I would wire the Volume pot like a normal Voltage divider instead to the 5E3 way that doesn't need mixing resistors for two channel operation. Note that no other Fender amp uses the Tweed Deluxe method of mixing channels.

If you will be using a separate input jack for each triode, then you will need mixing resistors between the caps and the Volume pot.

[Helmholtz]

Please post a drawing of yout intended wiring.

[Chuck H]

Hello and WELCOME.

Interactive 5E3 volume controls don't interest me, and I understand the compromises with ditching the low inputs and normal channel.

Do you? You want a 5e3. But you don't want a 5e3? It's precisely the weird wiring and interactive circuits that give that amp it's particular tone at different settings. This can't be achieved without other circuits that are equally as complicated (or worse) than just building a 5e3. Basically if you don't make it a 5e3 it's not going to sound like one. It'll be an amp. Possibly a good one.

I have read some older posts detailing stagger tuning a parallel triode from Bruce at Mission Amps, and want to try something similar.

Please post a link so we can all be on the same page regarding the circuits in question.

I have 33k grid stop resistors to each V1 grid, shared 620 ohm cathode resistor, 25uf/25v bypass cap, and two individual 100k plate load resistors.
For coupling caps I want to use different values, a .1uf and a .033uf, and run these to my volume pot.

So, not like a 5e3 at all then.?. I'm not trying to be a jerk here but I'm not going to walk lightly about it either because we actually do want to get to a result you'll be happy with. I'm not sure why you opened with saying you want to build a 5e3 when pretty much nothing about the build will represent that circuit. You could just post your proposed circuits and ask "Will this sound anything like a 5e3."

FWIW at the impedances in any tube guitar preamp circuit the difference between .033uf and .1uf values for coupling caps doesn't matter much. Blending the two in the circuit you propose won't present any sort of dynamic. In fact I'd be surprised if anyone could hear a difference between the two if they were used individually. The 5e3 has a dynamic via channel mixing only because of the interaction and filtering that happens when either channel is grounded through the other. Which is a pretty complex circuit actually. But you say you are not interested in it. If you want the amp to SOUND like a 5e3 I think you should be.

So here is my question, should I land my two coupling caps on the middle lug of the volume control or do I need mixing resistors for best results?

Like LT mentioned, there's no advantage to emulating the 5e3 volume control for a single output. That weird volume control wiring in the 5e3 eliminates the need for mix resistors AND is the cause of the interaction between the channels and their unique tonal properties AND is actually kind of clunky in it's function as volume controls. Without using another channel and without the TWO volume controls you don't stand to gain a thing to get you closer to a 5e3 by wiring the volume control that way. All you get is a clunky volume control and a higher than necessary noise floor.

What are you using for a phase inverter?

If you can't post a schematic for the amp you're starting with you'll need to be able to provide some detailed information as it's asked for. Like, what sort of power tubes are we dealing with? What is the plate voltage? Are you planning to rebuild the power supply to match the 5e3 or are you hoping to just tack a preamp in front of the existing power amp and supply?

You probably thought this was a straight forward question when you asked it. Sorry about that. It's actually very complicated.

EDIT: I couldn't find a legible schematic for the SC Muzak 1008 A or B. I could only decipher that it seems to have a suitable tube compliment and that Vp is 3XX (couldn't read it). And...

This is a three beer post. I do that sometimes and pick apart the problems with the problem as presented. But I stand by it because these are the things that turn projects into doorstops that never get completed. Sorry if I came across too snarky. My hope is that these problems can be exposed and solved for. So at least I'm well meaning

[Jazz P Bass]

Here is a copy of the 1008B schematic that I found.

[Chuck H]

That's great! Probably a lot of the circuit is similar enough, but I think the amp in question is the 1008A because F&A Electric had a 7199 for the first preamp tube. But hey, it's SOMETHING. And I can read it.

[Chuck H]

And here's what it looks like. We can see the 7199 tube is actually the one closest to the power tubes while the 12ax7 is closest to the input. So why was the 7199 socket rewired as the input tube? For best stability the signal should probably start with that far left 12ax7 for both inputs and rewire the 7199 socket for the shared second preamp stage and the PI.

You can actually use those vent holes in the back for mounting jacks and pots. I've done it. You just need some washers. It aint pretty but it's on the back. Then the whole 5e3 circuit could be used.

[F&A Electric]

Hi all, thanks for your interest and helpful replies. It’s been a busy weekend, and I’m going to try and answer your questions as best I can.

Firstly I am simply using the 5E3 as a basic topology to base a simple amp design on, not a clone and no expectations on what this will sound like, but I have a feeling it will sound pretty good. I feel like my title was misleading and I apologize for the confusion.

The output stage looked close enough, and the basic features of: 2 cathode biased 6v6’s, a 5y3, and two 9 pin sockets were all there. It even had a 250 ohm cathode resistor for the 6v6’s stock.
The phase inverter is the standard 5E3 cathodyne phase inverter.
The chassis had three holes for adjustment pots on the side to begin with, and I used two of those for input and output jacks.

To answer Chuck H on the tube arrangement, from input to output its 12AX7, 12AX7 (formerly 7199), 6v6, 6v6, and 5y3 rectifier. I can see why that was confusing since I said I changed it to a 12AX7 from a 7199. The two first preamp stages are assigned to V1 and the second preamp stage / phase inverter is V2.

Now to address the reason for only one volume and one tone. My favorite Fender amp is the 5F2-A and I wondered how a Tweed Deluxe would sound like set up with that volume and tone arrangement. I’m interested in trying it, no guarantee I will keep it like that.

It looks like I’ve got bigger issues however.

I finished wiring the circuit today, tried to bring it up on the variac and dim bulb limiter. With no tubes in the amp, at 75-80 volts the bulb was shining brightly and pulling half an amp. I heard what sounded like arcing inside the power transformer in time with the flickering of the bulb.

Jack Darr and Rob Robinette said to disconnect the secondary wires, so I did.
With only the primary side connected, the meter and bulb limiter showed the same condition.
It would seem that I built this project into an amp with a shorted power transformer primary winding.

I will evaluate this situation and try to find a suitable power transformer this week.
Maybe I’ll get by with a little help from my friends.

Thanks to Chuck H, loudthud, Helmholtz, and Jazz P Bass for the replies!

[F&A Electric]

Hi all, I wanted to update this thread as I have been hard at work on this project since my last post.
A good friend of mine came through with a late 50’s Stancor PM8409 power transformer and it was a direct replacement for my Muzak 1008.

I was able to complete the project yesterday evening, and felt that I should update my thread since I received such helpful feedback from the community here.
The amp works and sounds great, and I have done some experimenting with some of my ideas posted earlier.

The configuration that I ended up using was a single input jack feeding both grids of V1, and then volume controls for each channel.
i found this to be more versatile than the single volume control, and I much prefer being able to bring up the level of each channel independently.

I used a .1uf coupling cap for the normal channel, and a .033 uf coupling cap for the bright channel. Perhaps it is by virtue of the bright cap on the tone control, but there is a noticeable difference between the two channels and I am happy with the results. I also substituted the .1 uf coupling caps which are connected to the power tubes with .068 uf. I find the amp to have ample bass response, without sounding flubby in low frequencies.

Since both the bright and normal channels are always on, I don’t have the interaction between the used and unused channels that most people seem to love.
I am very satisfied with the sound however and enjoyed playing through this little brute for several hours last evening.

Thank you to all the contributors who offered suggestions and shared their thoughts regarding this project.
If anyone would like to share their favorite mods for the 5E3 I would love to hear them.

Warmest Regards - F&A Electric

[Chuck H]

How acout a schematic of your circuit?

[sparkies]

So why was the 7199 socket rewired as the input tube?

It was one of those common pentodes they used (usually as a phono or mic preamp). I'm surprised its not used in a Sony C800g cone since it is similar to a 6au6 (which the better one would be 7543)