Hiwatt Custom 50 DR504 Amp Mod On Slave Output

[TomCarlos]

Mates:

One of the amps in my friend's collection is a Hiwatt Custom 50 DR504 head. The amp is working but there is a modification that doesn't make sense. As of yet, I have not dived into looking to see exactly what is going on. But.... here is a summary.

The typical Slave output jack has been replaced with a pot. When you turn that pot, it affects the output level (on the far right output jack). I have the selector set to 8 ohms and using an 8 ohm cab. So it is as if that pot is tied into the preamp section somehow. That doesn't make much sense since this amp has pre gain and master volume pots.

But before I go diving into this amp, my question is this....

Looking at the schematic, it appears the Slave Output is being fed from the 16 ohm tap. Ig nothing is connected into the jack, you have a 3.9k plus 100 ohm resistor going to ground. But when you insert a cable, the signal goes through the 3.9K through the cable and returns to ground. Is that it? So my assumption is that if someone did not use the Slave Output they could either (1) leave the 3.9K and 100 ohms connected into the circuit with no jack or (2) remove this circuitry and be done with it.

Schematic - Hiwatt_50w_dr504_Schematic.pdf

Thanks for the advice....

[Jon Snell]

I presume one of the speaker jacks is being used as the slave out?
Slave out is normally a line level signal but from your description it is now variable.
This system is the cause of many headaches in past years for us sound engineers. The number of earth loops was a constant problem.
Ampeg used an isolation transformer and that solved a host of issues.

Sorry but I cannot view your photos at present as my browser is having problems with visual basic 5.

[Leo_Gnardo]

My best guess is that the second speaker output has been reassigned to a variable level slave out. You'll have to crack open the Hiwatt, trace some wiring to the added pot & see whether that's the case or not. As far as the original slave out, the 3K9 and 100R form a voltage divider and present a signal 1/40th of the voltage on the 16 ohm winding to the marked slave out.

Hmm... is it no longer pee cee to use the term "slave out" ??? Jeepers, ya gotta be careful what ya say these days! Don't need to start a demonstration oh please NO!

[mhuss]

On the ones I've seen, it's just a simple voltage divider to take the speaker output down to something in line level territory. The divider is always in place if a jack is inserted or not.

[Chuck H]

I'd be inclined to restore the stock circuit. That way whatever that pot is doing now won't bugger and complicate the preamp function anymore and both speaker jacks are back in place. JM2C

[TomCarlos]

Thanks mates.... I will try to get to this tomorrow.

I am dang curious to see where the pot ties into. If I can disconnect it, I will (while making sure it does not disrupt the circuit.

My guess is that the Slave output was built using a Cliff jack, S-H952, mono 2 pole, or something that is normally open? I found a photo on the web showing this, although we cannot see the details. But for now, the important thing is that the first two jacks should be connected in parallel. After that, a 3^rd (Slave Output) jack is wired as shown on the schematic.

[g1]

It sounds like you had a speaker connected to that jack and the pot was controlling the output level to that speaker? Probably at low power levels?
And the jack that the pot was controlling was not the jack directly beside it? That seems a bit odd, also that they did not label the jack as being non-speaker output.
Hopefully you can draw out what you find, or post some pics.

I guess another possibility would be some kind of variable NFB control.

[vintagekiki]

... modification ...

The implementer of the modification got lost somewhere in his idea.
I guess idea should look like ...

[TomCarlos]

Ok…. I think I have it (with emphasis on think !!!)

Now the problem once again – you turn that pot and you lose all output. The output is also very noisy (but that might be a different problem).

And I am “thinking” I may not have the correct schematic because the parts I see do not match the output section. So maybe it is the wrong version or something.

I am only showing 1 side of the mod. The same mod is done for the lower EL34 tube.

The two output jacks remain connected in parallel and they are NOT connected in any way to that pot.

The pot is a stereo, 250K.

I do not see a 1K screen resistor. I see a 100 ohm, 1 watt. One of the resistors looks a bit toasted but it still reads 100 ohms.

The 22K Grid resistors are still there.

I do not see 100K resistors connecting to the bias. I see 220K.

I do not see the original .047uf grid caps. I see .022uf caps (the mustard caps). It looks like they belong there.

The mod also uses .1uf orange drop caps.

One end of the .022uf has been lifted from the PC board (the end that went to the 22K) and one end of the .1uf cap was soldered where the .022 resided (going to the 22K resistor). The other end of the 1.uf goes to the center of the pot.

One end of the .022uf goes to one end of a 250K pot and the other end goes to the PI circuit (page 2 of the schematic).
The other end of the pot goes to ground.

The .1uf cap connects to the connection point of the 22K and 220K.

The 100uf cap, diode, 100K, and 27K resistor are the bias circuit (see page 1 of the schematic).

This does not look like negative feedback to me. Maybe it is some type of Presence control?

So it appears that the easy fix would be to remove the Orange drop, solder the .022 back where it was, remove the two coax cables and 250K pot.

As far as the Slave output is concerned, if you look at the other photo, it appears as if the 3.9K and 100 ohm resistors are connected right on the Slave jack (and not par of the PC board).

I gotta belief the circuit was built with .022uf grid caps. But what about the 100 ohm screen resistors?

There you have it…

[vintagekiki]

not have the correct schematic

What you are describing is most likely the master volume in PI for the purpose of pre-allowed (preset) max power.
Similar schematics used by Orange, Trainwreck and some other amps.
View and compare attachments, then draw the actual state power amp.

https://music-electronics-forum.com/forum/amplification/guitar-amps/theory-design/43144-train-wreck-master-volumes
Train wreck Master Volumes?


https://el34world.com/Forum/index.php?topic=21683.0
Orange master-volume installation

[Leo_Gnardo]

There you have it

Sooooo much easier with a schematic - that's a PPIMV - Post Phase Inverter Master Volume. Yes, you would expect volume to drop to nothing when dialed all the way down.

[TomCarlos]

Thanks guys....

So, a new term to my amp vocabulary... "Post Phase Inverter Master Volume."

Once I saw VK's post #10, I realized I drew it correctly. And thanks VK, for the links to the other amps using similar mods.

I think I am going to yank out the extra parts, solder the .022uf caps back in, and fire up the amp to see what it does. I was noticing some noise previously ... maybe a dirty tube socket, weak tube, maybe from this mod, we'll see.

The other thing that is odd on this amp is a type of "rod" that runs from the output jacks to the Impedance selector. I guess it is a safeguard to prevent you from changing the tap on the OT while you're playing. So if you want to use a different cabinet, it looks like you need to unsolder the rod, change the selector to a different tap, then resolder the bar. Even though the knob has a spot for 4 ohm, I only see two green and two black wires on the back of the selector. So those must be the 16 and 8 ohm outputs only as shown on the schematic.

[vintagekiki]

... output jacks to the Impedance selector ...

A "weird" way connect the output jacks to the impedance selector is to disable any amp operation without the speakers connected.
Without the speakers connected, the OT is protected (short-circuited), and less likely to be damaged (burn between the primary layers).
When speaker cabs working in pair, the output jacks are connected in parallel.

[Leo_Gnardo]

Thanks guys....

So, a new term to my amp vocabulary... "Post Phase Inverter Master Volume."

Once I saw VK's post #10, I realized I drew it correctly. And thanks VK, for the links to the other amps using similar mods.

VK for the win! Beat me to it by a minute, and with a clear PPIMV schematic besides.

The other thing that is odd on this amp is a type of "rod" that runs from the output jacks to the Impedance selector. I guess it is a safeguard to prevent you from changing the tap on the OT while you're playing. So if you want to use a different cabinet, it looks like you need to unsolder the rod, change the selector to a different tap, then resolder the bar. Even though the knob has a spot for 4 ohm, I only see two green and two black wires on the back of the selector. So those must be the 16 and 8 ohm outputs only as shown on the schematic.

That "rod" is just a piece of bus wire connecting the speaker "hot" to the selector. Normally no de-soldering is needed to change impedances. And you found the Hiwatt cheat - there is no 4 ohm tap on the OT. 4 is run off the 8 tap for better or worse.

[vintagekiki]

... outputs only as shown ...

Early 70's Hiwatt amps had OT with taps for all ohms.

Early 70's DR103 layout COMPLETE, rev 1.0 (PDF)
Early 70's DR504 layout COMPLETE, rev 2 (PDF)
DR 103 Manual (Complete)


https://hiwatt.org/index.html

https://hiwatt.org/HiwattStory/