Valve PA amplifier - what to do with it?

[peacocksuit]

Would it be worth considering a separate transformer for the heaters?

 

[trobbins]

My query is does the vibrator go to a separate set of transformer windings than what has been shown in schematics so far.

 

[OldHector]

What is it? Philips EL6415 or EL6425? I've got two of the latter. Much more inconvenient output impedances, btw

Hi Kay, sorry for the late reply, EL6415. Do you have any plans to use yours in anyway?
Cheers, Richard

 

[peacocksuit]

My query is does the vibrator go to a separate set of transformer windings than what has been shown in schematics so far.

Again, frustratingly I'm not in front of the amp at the moment. I will be able to have another good look over it tonight. What I can definitely tell you from memory is that the vibrator socket has one wire to pin six on the transformer (I may have mistakenly said pin 7 in one post), and another wire goes to the ten-pin power socket. One of the pins from the power sockets also went to the battery stand by switch. I believe the outgoing side of the standby switch may have fed into one of the primary mains transformer pins, I will have to check this later to confirm. Pin 2 of the EL34 was also connected to one of the pins in the power socket.

I guess the plugs we're wired to make connections between some of these pins depending on whether the mains plug or the battery source plug was connected.

 

[OldHector]

You mentioned there were alternate primary winding taps depending on input voltages, but you did not show them in your table of resistances. Have you tested all resistances between all connections of the transformer?

 

[peacocksuit]

The table if resistances was just on the secondary side, Hector.
I will repeat this test later today with an updated table showing all transformer connections

 

[Kay Pirinha]

Hi Kay, sorry for the late reply, EL6415. Do you have any plans to use yours in anyway?
Cheers, Richard

No, not yet. As the input and output terminals are more than unusual, as there's only one bias voltage for all four EL36's that isn't even adjustable, as the speaker outputs aren't too usable, and as the output transformers are mediocre at it's best, I think a complete redesign, including rewinding the transformers, would be needed to get them into home hifi service.

Anyway, I like that typewriter like appearance!

Best regards!

 

[peacocksuit]

My query is does the vibrator go to a separate set of transformer windings than what has been shown in schematics so far.

MT primary pin 6 had a wire going to the socket, the vib socket had a wire also going to he socket. So perhaps indirectly via the socket pins?

Pin 7 MT primary has a wire which goes: pin 7 - fuse - standby/battery switch - 10 pin socket. Or probably reverse order of that sequence to make more sense!

 

[peacocksuit]

1PR

2PR

3PR

4PR

5PR

6PR

7PR

1SE

2SE

3SE

4SE

5SE

6SE

7SE

1PR

/

8.4​

8.6​

9.3​

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

2PR

8.4​

/

0.9​

1.8​

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

3PR

8.6​

0.9​

/

1​

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

4PR

9.3​

1.8​

1​

/

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

5PR

N/C

N/C

N/C

N/C

/

0.1​

0.3​

N/C

N/C

N/C

N/C

0.4​

0.3​

0.3​

6PR

N/C

N/C

N/C

N/C

0.1​

/

N/C

N/C

N/C

N/C

N/C

0.3​

0.2​

0.2​

7PR

N/C

N/C

N/C

N/C

0.2​

0.2​

/

N/C

N/C

N/C

N/C

0.2​

0.2​

0.1​

1SE

N/C

N/C

N/C

N/C

N/C

N/C

N/C

/

74.6​

79.5​

NC

NC

NC

NC

2SE

N/C

N/C

N/C

N/C

N/C

N/C

N/C

74.6​

/

79.5​

NC

NC

NC

NC

3SE

N/C

N/C

N/C

N/C

N/C

N/C

N/C

79.5​

79.5​

/

NC

NC

NC

NC

4SE

N/C

N/C

N/C

N/C

N/C

N/C

N/C

NC

NC

NC

/

NC

NC

NC

5SE

N/C

N/C

N/C

N/C

0.3​

0.4​

0.2​

NC

NC

NC

NC

/

0.1​

0.2​

6SE

N/C

N/C

N/C

N/C

0.3​

0.2​

0.2​

NC

NC

NC

NC

0.1​

/

0.2​

7SE

N/C

N/C

N/C

N/C

0.2​

0.2​

0.2​

NC

NC

NC

NC

0.2​

0.2​

/

 

[peacocksuit]

That's all the pins on the transformer.
There may be one or 2 which are slightly different one way than the opposite which should really be the same. This is just because I measured some of them twice rather than just gone off the previous combination of the same test, and there's a slight variation.
If that makes sense.

That's every possible combination of resistance between all the pins though.

 

[trobbins]

Can you confirm that 5PR, 6PR and 7PR have not been included in any of your sketch schematics to date? If not then that group of taps appear to be the 12V push-pull winding used by the vibrator to act as an alternate form of MT energisation (in lieu of the normal mains primary on 1PR to 4PR taps). I also don't see any measurements of voltage for those 5PR to 7PR taps (to compare with previous test voltages in say post #112) - which would also indicate which of those taps is the CT tap for that group, and hence confirm the wiring arrangement for the front panel octal socket.

It may well be that 5PR to 7PR could be a better means to power a 12V heater circuit (as per how the valve heaters are wired up now).

 

[peacocksuit]

I did briefly test primary 5-6 6-7 7-5 but no combination give 12v. I will do it again and record the results properly.

 

[peacocksuit]

Tried the opposite side of the transformer (P5, P6 and P7), no combination of these give me 12v either.

I decided to try a separate transformer I had to temporarily supply the heater circuit, it's heavy duty toroidal good for 6.5a. 2 separate 12v outputs. With the heater connections to the MT disconnected, I connected one end of 2 of the 12v windings one to ground and one side to pin 2 of the EL34. Almost straight away the heaters of the EL34 and EZ81's valves are glowing. Not the pre amp stage valves though.I
At this point I'm now seeing the bulb lighting up on my dim bulb tester. Doesn't happen when the 12v transformer is switched off, only when the heaters have power. It's a 100w bulb and I wouldn't say it's on fully, but it's lighting up.

Still no sound whatsoever from the speaker I have wired up.

So now I've decided I'm going to just rewire the heater circuit from scratch and I've got a 240v to 6v-0-6v transformer rated 8A to try.

Im thinking one of the 0v-6v outputs supplying the rectifiers and EL34'S (in parallel), the other supplying a bridge rectifier for the pre amp tubes, assuming DC to be the better option for these with regards to hum? May need dropping slightly to bring it back down to 6v(6.3v+/-)?

Would it be better to stick to 12v series supply for the EZ81's and EL34'S, making use of the CT on the new transformer? If I go ahead with parallel for these tubes, would I need to ground the 0v pin?

How do you think this configuration would work out?

 

[trobbins]

For interest, what were the measured voltages between P5, P6, P7 ?

Imho, it is better to supply the rated voltage to each heater (ie. yes use a 6-0-6 by using the CT to effectively separate the windings), so not in series as that requires both heaters and interconnects to be functioning nominally, and that may not always be the case with different manufacturer valves. So yes the simplest way is to ground the CT of that winding, but that may not be the lowest hum setup - but you won't know that until all the gremlins are out of the way and the amp is in to fine-tuning stage.

 

[peacocksuit]

Finally....some sound out of this old thing!

I ended up sticking a 6v-0-6v transformer in there to run the heaters and I've re wired it for parallel.

First power up, everything glowing nicely but no sound at all from the speaker.
Noticed the ECC81 wasn't glowing, turned out there was no continuity between the heater pins.
So I replaced both of the ECC8x valves with a pair of 6N2P's and made the necessary pin changes at the valve based. And then...., sound!

Lot of buzzing from the speaker, I haven't grounded the CT on the new trafo, I'm going to ground it, I'm hoping this is the cause. I am thinking a bridge rectifier to supply the input stage heaters with DC might improve the buzzing if it doesn't go away once I've done as above.I 'm also not sure if the way I've wired up the valves using different sides of the trafo is an issue, I've put the EL34's and rectifiers on one half and the preamp valves on the other side.
I've also taken 4 twisted pairs to each pair of valves with a parallel link between the 2 valves in each pairing.

It's not as loud as I expected but there are a lot of resistor values which have drifted which and need to be replaced. I only tested it briefly but it was though a 4ohm speaker (all I had to hand). Perhaps this is playing a part?

Plan now is to check the voltages throughout the circuit and the output valves biasing.
I'm going to get an 8ohm speaker to use and maybe use a dummy load on the speaker output whilst doing further testing.

I'd like to have gain control, I have 3 spare pots, 2 from the redundant mic stage and the second gram input. I'm thinking of wiring it for a dual impedance input like the fender amps using 2 jack sockets for the same input.
I'm thinking I'll just copy a gain control circuit from another schematic?

Reverb would be fantastic but I'm not sure I've got room for a tank. How difficult would it be to add this to the existing circuit?
I'd love tremolo as well and have spare valve sockets as the mic stage ones are now redundant.

I was wondering about the 2 ECC8x valves I've taken out; one of these was purely for amplification and the other a phase inverter.
Are the 6N2P's I've replaced these with suitable for both tasks? Presumably some tweaking of the circuit around these will need adjusting to make these work to their given specifications.

Finally, there's some noise coming from one of the valves. It's difficult to pin point whether it's the EL34'S or the rectifiers as their in real close proximity. It's like a very quiet rattling/buzzing. Is this a concern?

Thanks again for your input. I'm really happy to have got to this stage!

 

[peacocksuit]

Haven't had much time to work on this for the past week unfortunately. I've had a bit of a look today. I'm looking at known guitar amp schematics and thinking about copying the input stages to suit 12ax7's as I have a load of them and it makes sense given their so commonly used in so many amps. I've looked at some marshall schematics for 50w amps which use 2 EL34's for the power stage.

I looked at this one

It's really useful as it lists the expected voltages.

So I had a look at the voltages I have. I disconnected the pre amp stage nodes from the HT supply as measured what I had

Even before the resistors I'm only seeing 356v.

On the JCM800 schematic, the voltage at a similar point, before any dropping resistors or valve load seems to be 465v.

The EL34's circuits where still under load when I measured this but the nodes supplying all the pre amp valves where disconnected from HT supply. So I'm only getting 365v before it's seen any load from the pre amp stage.

Is the Marshall mains transformer just higher output than mine, or is there something else at play which I'm not seeing?

My MT disconnected under no load puts out >700v AC

I looked at the specs for a MT designed for a JCM800 50w

Looks like the output should be pretty similar to mine? I have no idea of the power rating of my transformer though..

I see diode rectification on the marshall circuit. Am I seeing the effects of drop caused by the tube rectifiers?

Thanks as always for your input. It's much appreciated 👍

 

[OldHector]

You are at a crossroads, the same one I have been on a few years ago. I tried like you to reverse engineer a PA amplifier, and by not being careful enough with my approach and pictures, I got to the point where I could no longer describe the original circuit. At that point I had to start looking at solutions for the same tubes an operating points to reuse as much as possible.

My suggestion of getting to know LTSpice earlier might have helped you capture the old circuit by being able to emulate it is that still possible?

If not then what you are doing is about the only option, i.e. start from scratch with a similar count of similar tubes.

That dual power supply and vibrator made it complicated.

 

[peacocksuit]

Hello, mate. Hope you are well. I definitely want to learn LT spice. I just don't have the time at the moment with work and life and stuff. It seems like quite an undertaking. At least for now, when I get a spare hour here and there to take myself off into my shed I'm able to tinker with it and get hands on, which is a pleasure to be honest, never a chore in the slightest. I'm still watching videos and reading stuff on the theory, bit by bit I'm learning all the time.

I could easily put it back as it was, replace the original valves (replacing the broken one like for like). But any time I spend trying to make it work in its original configuration seems a waste really, given that it was always my intention for it I become a guitar amp anyway. I have no use for a valve PA. I want a guitar amp that's half decent. Sure, It will amplify a guitar signal in its original intended config. But it won't do it as good as a circuit intended for that purpose specifically.
The input stages in a guitar amp (I know you know this, not trying to teach anyone to suck eggs here...) are designed and biased to give nice tones when clean or distorted and to give the player control over the sound. As a PA, it's designed for microphones and a gramaphone, it's totally different.

But I have (or should have) all the necessary major components here to remake it into something useful. And something I'd get a lot of use out of hopefully.

I understand some people might think it a shame to do that to it. But, at the end of the day it's not a museum piece. If I sold it (which I never would) it's only monetary value would be the sum if it's components anyway. I can't imagine it having much use to anyone as a PA. Since I've had it in my possession it's spent most of its time in a loft. It's subjective, but I think it will make a cool guitar amp. I'm not altering it's outward appearance at all, so I think this is the best thing.

That dual power supply and vibrator made it complicated

Definitely ☹️

 

[trobbins]

Perhaps the basic operating levels of the power transformer need to be confirmed, and included at the start of an inquiry (given this thread is getting quite long now), before trying to make comparisons or asking for advise - especially for new eyes.

For starters, the heater voltage, relative to the primary winding tap used needs to be settled imho. Then with the given primary tap, the HT voltage can be confirmed (ie. is it going to be 375-0-375 or 415-0-415V) ? Then the primary and secondary winding resistances need to be identified - the secondary resistances were 75 + 75 ohm in an earlier post - then forum viewers can use PSUD2 to indicate whether the EZ81's are operating within spec, and whether the 150 ohm anode resistors are needed or can be reduced.

Then you need to confirm the idle bias power dissipation of the EL34's is appropriate, and confirm they are all balanced and drawing an acceptable current (for the B+ voltage you have available), so that we know what the main loading on the power supply is (ie. the preamp stages draw very little current compared to the output stage). They all impact the likely B+ you will end up having, and using for comparisons. Just my 2c.

 

[peacocksuit]

Hello, again Trobbins. Your 2c is always gratefully recieved.Thanks for the reply. Just had a read through the above

For starters, the heater voltage, relative to the primary winding tap used needs to be settled imho.

I'm at a loss with that to be honest. I admit I have no idea how to proceed there. If anyone's willing to help, of course I'd appreciate it. I listed all the resistances in a table earlier on in the thread. I give up in the power transformer for heater supply in the end as I say, I couldn't find any output that would have worked with a series wired heater circuit. If it's still relevant to understanding the transformers characteristics in a broader sense, I can go back to it. But I have to admit I was struggling with that.

Then with the given primary tap, the HT voltage can be confirmed (ie. is it going to be 375-0-375 or 415-0-415V) ?

It's currently using the 240 (220-240 i think) winding

Then you need to confirm the idle bias power dissipation of the EL34's is appropriate, and confirm they are all balanced and drawing an acceptable current (for the B+ voltage you have available), so that we know what the main loading on the power supply is.

I will upload the results of this tomorrow. I have this recorded. I'll probably repeat the test just and compare the results to be sure.

Thanks again 👍