Hiwatt Custom 50 DR504 Amp Mod On Slave Output

[TomCarlos]

Thanks again guys....

I don't want to beat this to death, but of all the schematics I have seen, none are an exact match for the DR504 that sits in front of me. No super biggie...

By the way, going back to my hand drawn schematic in post #9, what is the official term (or name) of the 200K resistors that are feeding the 22K grid resistors?

[Helmholtz]

.
By the way, going back to my hand drawn schematic in post #9, what is the official term (or name) of the 200K resistors that are feeding the 22K grid resistors?

They are typicall called "grid reference resistors" as they reference/connect the power tube grids to the bias voltage.
You could also call them grid leak resistors as they provide the necessary path for grid leak currents, just like the grid leak resistor of a preamp tube that goes to ground.

[vintagekiki]

... hand drawn schematic ...

DR504 OL
without MV in PI

[TomCarlos]

DR504OL - That is the one !!!!

[TomCarlos]

Epilogue - After removing the mod and tacking the .022uf back into place, the amp sounds great !!

Many thanks for the help with this amp!!

Tom

[Chuck H]

Epilogue - After removing the mod and tacking the .022uf back into place, the amp sounds great !!

Many thanks for the help with this amp!!

Tom

Not to doubt that that the amp sounds great as stock and without the mod, BUT...

This being an amp "in your friends collection" doesn't speak much to how your friend uses it. Your experience with the post PI master volume was that it acts as a secondary volume control. If you only tested the amp up to max clean output levels then the post PI master would indeed seem to have no useful purpose. But it's actual intended purpose is to allow the amp to generate distortion BEHIND the master volume so that the preamp can be cranked up into the PI and all those distortions may then be dialed down to the desired level by the master volume. This is to say that if your friend was actually using the post PI master volume for it's intended purpose then your friend may find the amp is now missing a feature that they need in order to use it in a familiar way.

Just thinking out loud. I'm surprised this wasn't addressed earlier and I apologize for coming back in late to point this out.

[mhuss]

The OL amps have a lot of preamp distortion on tap - nothing in Rectifier territory, but certainly JCM800-ish. Since the master volume is right before the PI, I don't know that overdriving the PI would add a lot to the stew.

[nevetslab]

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.

I've never seen any guitar amps with a 100V output tap on them. Not unusual in distributed sound systems having 70V taps. Think of the fun you can have if your guitar tech keeps messing up, and have him hold the end of THIS cord plugged into that!!

[vintagekiki]

I've never seen any guitar amps with a 100V output tap on them.

Old Hiwatt and Sound City amps had an OT tap for 100V

https://www.tubeampdoctor.com/en/output-transformer-for-hiwatt-custom-100-dr103-tg5479
Output Transformer for Hiwatt Custom 100, DR103, TG5479

http://soundcitysite.com/B100_schem.jpg

[Helmholtz]

100V (not 70V) sound distribution systems were a standard in Europe in the 50s and 60s. Several 60s European guitar amps used an OT with an additional 100V secondary (not actually a tap), probably because they could use the same OT also for their PA versions, where the 100V output was considered a desirable feature.
E.g. early Marshall 50W and 100W Drake OTs.

[Leo_Gnardo]

Several 60s European guitar amps used an OT with an additional 100V secondary (not actually a tap), probably because they could use the same OT also for their PA versions, where the 100V output was considered a desirable feature. E.g. early Marshall 50W and 100W Drake OTs.

I have seen the 100V output feature on some early UK guitar amps. Same companies (I'm guessing Marshall, Laney, Sound City, Hiwatt) typically made rudimentary PA heads with 4 input channels. How it was used on concert PA's is anybody's guess. Typically there's a stepdown transformer for each speaker. More complexity, but who knows maybe some were operated that way. These kind of distros are common to large stores, factories & office buildings. I can just imagine strolling through a big multi level department store and through all the PA speakers in the ceiling comes 100 watts of : "Attention customers! Now hear this! Blue light special right now on . . . KERRANGGGGGGGGGG!"

[Chuck H]

I have seen the 100V output feature on some early UK guitar amps. Same companies (I'm guessing Marshall, Laney, Sound City, Hiwatt) typically made rudimentary PA heads with 4 input channels. How it was used on concert PA's is anybody's guess. Typically there's a stepdown transformer for each speaker. More complexity, but who knows maybe some were operated that way. These kind of distros are common to large stores, factories & office buildings. I can just imagine strolling through a big multi level department store and through all the PA speakers in the ceiling comes 100 watts of : "Attention customers! Now hear this! Blue light special right now on . . . KERRANGGGGGGGGGG!"

I contracted at an "elderly living facility" painting the apartments. They had an old tube PA system with a 70V line transformer run to each of three floors for 168 units. Every now and then the old 10" full range speakers (in wooden boxes on the walls) would distort a little. I once removed one for painting and researched the speaker code. They were made by Magnavox. Maybe a combination of the PA and the speakers distorting? But it sounded "right". I remember thinking that it might sound pretty good with a guitar cranked through it Just idle thoughts while painting apartments for retired people. Some might have enjoyed it. Most, not so much.

[Chuck H]

The OL amps have a lot of preamp distortion on tap - nothing in Rectifier territory, but certainly JCM800-ish. Since the master volume is right before the PI, I don't know that overdriving the PI would add a lot to the stew.

I went ahead and did a cad simulation of the amp. I have to say that the signal coming out of the regular master volume off the tone stack is pretty goofy. I'm not at all sure it would be easy to reconcile with eq adjustment. But after going through the PI the wave form squares off much better and I expect it would be more to any players liking. Of course the cad doesn't take some things into account like capacitances due to layout and shielded cables, etc. But there's definitely more going on that is akin to what a player expects after the signal is processed through the phase inverter.

[Leo_Gnardo]

I contracted at an "elderly living facility" painting the apartments. They had an old tube PA system with a 70V line transformer run to each of three floors for 168 units. Every now and then the old 10" full range speakers (in wooden boxes on the walls) would distort a little. I once removed one for painting and researched the speaker code. They were made by Magnavox. Maybe a combination of the PA and the speakers distorting? But it sounded "right". I remember thinking that it might sound pretty good with a guitar cranked through it Just idle thoughts while painting apartments for retired people. Some might have enjoyed it. Most, not so much.

Magnavox ... was founded by Peter Jensen. So those humble distributed speakers were more or less Jensens, a treasury of them sprinkled throughout that building. And so many other buildings besides. Your ears weren't lying! A true test would have you plugging your guitar into the PA amp and serenading the residents. Well maybe not, might have brought on a few heart attacks... If they haven't yet got rid of them, I'd go back there and ask that you be called in to gather those old speakers should they decide to modernize their distro PA.

[Chuck H]

If they haven't yet got rid of them, I'd go back there and ask that you be called in to gather those old speakers should they decide to modernize their distro PA.

It had occurred to me. But this place is in San Jose, Ca. and, unfortunately, I no longer have an "in" there as they changed management and have moved on to another painter since my move to the PNW. They wouldn't know me from Adam if I walked through the door now.