I have a valve PA amplifier which has been my possession for many years. It was my late grandfathers. I've never seen it working. It's complete but would obviously need a fairly comprehensive overhaul before it could be used. The valve line up is EL34, EL34, ECC81, 6BR7, 6BR7, ECC82. There are 2 EZ81 rectifiers also.
It's an strange old thing which has the option of running from a 12v battery. It has a wierd mains/battery socket on the front. I think it looks pretty cool and would love to use it as a guitar amp and build a little cab for it to sit on top of, finishing both the amp and speaker cab in a matching quilted leatherette similar to the beaten up stuff that's already on the amp casing.
Whatever happens, it stays as it looks now. It won't be cannibalised for parts!
The issue is that I can't find a schematic for it. I've looked many times over the years and come up with nothing.
Searching on Google just brings up old posts on vintage-radio, (one of which was made by myself), another gives some interesting background info on the company who made it. However there's no schematics posted anywhere.
If I had a diagram to follow I'd be able to make better sense of the circuit but as a relative beginner, it's beyond my capabilities to trace out the entire circuit.
So, I'm thinking of just starting from scratch and finding a suitable guitar amp schematic which uses the EL34's and rewire it that way.
I've no use for the 12v circuitry etc...
I'm thinking a good starting point would be to make sure the transformers work, these being the most expensive things to replace.
If anyone can suggest a guitar amp that could be make use of as many of the existing components as possible, let me know.
Any ideas on how to precede will also be welcomed!
This should be posted on the instruments and Amps forum
https://www.diyaudio.com/community/forums/instruments-and-amps.30/
I'd have no hesitation in converting this into one of the many Fender guitar/bass tube amps. Huge amounts of help everywhere for these famous circuits, pcbs, YT videos, everything. Rob Robinette is essential reading.....
https://robrobinette.com/Amp_Stuff.htm
https://www.diyaudio.com/community/forums/instruments-and-amps.30/
I'd have no hesitation in converting this into one of the many Fender guitar/bass tube amps. Huge amounts of help everywhere for these famous circuits, pcbs, YT videos, everything. Rob Robinette is essential reading.....
https://robrobinette.com/Amp_Stuff.htm
Thanks, Any. In not familiar with Robs stuff, had a quick browse then and it looks like a treasure trove of info! I'll get stuck into that.
Re the appropriate section for the post, atm it's just a PA amp and not a guitar amp which is why I chose this section. Perhaps it would attract more interest in the instruments and SNPs section?
Re the appropriate section for the post, atm it's just a PA amp and not a guitar amp which is why I chose this section. Perhaps it would attract more interest in the instruments and SNPs section?
Why do you think you can’t draw your own schematic? If you have a multimeter that can measure resistance you can do that even if you can’t read the color codes. The only caveat is to be sure what the component actually is. Sometimes a capacitor can look like a weird resistor.
The meter will also let you identify transformer leads that are attached to the same winding.
Just start with one input tube.
The meter will also let you identify transformer leads that are attached to the same winding.
Just start with one input tube.
I could theoretically trace it all out, yes. That was my original intention. I was going to keep the original circuit and just replace the capacitors and then see how it worked as a guitar amp. In which case, I suppose tracing out and drawing the circuit would be the way to go. However, as I'm hoping to use it as a guitar amp, it seems a bit pointless to do that as it might not make a great amp for guitar in its original configuration.
I could of course be wrong about this and it might work well as a guitar amp 'as is'.
Perhaps people with more understanding could provide input on that?
I'm a drummer and not a guitarist so my understanding of what features make for a good amp are also someone limited 😁.
From my understanding though the 2 output tubes would provide plenty of power. I want to be able to overdrive it etc.
I own guitars and can play but I've never played guitar in a band or anything. It's not my main instrument. Would just be nice to make a decent amp just as something to have.
I could of course be wrong about this and it might work well as a guitar amp 'as is'.
Perhaps people with more understanding could provide input on that?
I'm a drummer and not a guitarist so my understanding of what features make for a good amp are also someone limited 😁.
From my understanding though the 2 output tubes would provide plenty of power. I want to be able to overdrive it etc.
I own guitars and can play but I've never played guitar in a band or anything. It's not my main instrument. Would just be nice to make a decent amp just as something to have.
It appears to be circa 1960, based on the Hunts code of HAA. I'd recommend putting a post on https://www.vintage-radio.net/forum to see if anyone knows of the maker, and or manual/schematic. Sometimes information can come in after months/years - so if you're not in a hurry then perhaps take your time to collate information.
The basic circuit is unlikely to be difficult to reverse engineer for someone who has worked on PA amps, but yes you may find that daunting. At least it has common speaker impedance taps to make that aspect easy.
One detailed link that may or may not give you an indication of what you would be getting yourself into if you wanted to restore it yourself is: https://dalmura.com.au/static/Renovating PA amps.pdf
The basic circuit is unlikely to be difficult to reverse engineer for someone who has worked on PA amps, but yes you may find that daunting. At least it has common speaker impedance taps to make that aspect easy.
One detailed link that may or may not give you an indication of what you would be getting yourself into if you wanted to restore it yourself is: https://dalmura.com.au/static/Renovating PA amps.pdf
I'm having a go at drawing the circuit out today. Well, making a start anyway.
I'm a bit confused by the wiring at this valve base. In the socket is an ECC81 dual triode.
I thought a B9A socket had pin 1 bottom left and pin 9 bottom right going clockwise as seen here
Bellow, pin 1 appears unused and pin 6 appears to be grounded.
I expected to see high voltage connections at 1 and 6?
I'm a bit confused by the wiring at this valve base. In the socket is an ECC81 dual triode.
I thought a B9A socket had pin 1 bottom left and pin 9 bottom right going clockwise as seen here
Bellow, pin 1 appears unused and pin 6 appears to be grounded.
I expected to see high voltage connections at 1 and 6?
I've had the valves out of this but was careful to put them back in the same sockets. I suppose at some point in its history it could have had them switched while not in use...
Perhaps the 6BR7 should be in this socket?
Perhaps the 6BR7 should be in this socket?
This might make more sense as pin 7 is the anode. 7 seems to have the HT voltage and also a capacitor (blocking) also connected.
2 would be the grid? Which is where the input seems to be wired to...
Plus there are 2 6BR7's, each of them seems to be involved with the 2 gram stages with the 2 gram input jack sockets each wired to one of the first 2 sockets. It would seem to suggest the first 2 sockets are 6BR7? Pin 1 is left out in both...
Thanks Trobbins. There are posts on vintage radio including one by me from quite a.
while back. Another member posted theirs which they got working. No wiring diagram unfortunately..some info on there about the company though which is interesting. Other than that there's not much out there all all.
There is couple of old listing on the site reverb for hifi ltd amps, a senior model and a junior, which I think mine is.
This is listing for the senior:
https://reverb.com/uk/item/2687707-...uk-made-2xel34-amp-super-clean-serviced-ready
I'm enjoying looking more closely at the wiring and I think it's a great learning opportunity.
Now I've made a start I'm finding it a bit less daunting. Even if I rewire it, it would still be nice to have a working diagram for the original circuit.
I think it's a good idea to convert it into a guitar amplifier. You don't even need to trace down the original schematics for this purpose. Fender schematics already have been suggested, but as there's a pair of EL34's, rather Marshall or Hiwatt come to mind. You might want to have some look at Mark Huss site for Hiwatt schematics or schematicheaven.net for Marshall.
Best regards!
Best regards!
Certainly seems that is the correct interpretation, given the 6BR7 would have been an appropriate design choice for the input stages (albeit not a commonly used valve).
I'm all for supporting your effort to document what you have first. I have restored 50 or so valve amps, and the vast majority have been PA amps, and typically each different model provides a new insight into either an historical period/company/use and/or a technical aspect. There were just so many made around the 1950-70's. I often do a draft schematic sketch, and then redraw it a few times as the general connectivity and staging becomes more apparent and all the wiring is accounted for - especially where the circuit may be somewhat uncommon, like around tone control stages, or stages that use local feedback, or use an uncommon phase splitter or mixer, and if a dc input with vibrator is used with a common power transformer.
I have ended up restoring most amps to be useable for guitar/bass, but imho that choice comes after an appreciation of what you've got, and often after testing certain parts to confirm you aren't about to start a restoration with bad key parts like transformers. Also there is no 'standard' guitar amp circuit, and often a PA amp requires very little change in core stage circuitry to achieve a useful amp with practical gain stages that allow overloading and tone management.
Your amp photos indicate a nice level of british construction. Most of the resistors will have drifted +10 to +20%, but per se that is not likely a pre-requisite for initially changing them. But most/all the Hunts capacitor would require replacement, as even service departments in the 1960's were recommending their replacement for 1950's gear. So luckily it was your grandfathers, and you can enjoy making a journey that is not all about simply financial gain and expedited effort or to make a predetermined clone.
If you post a photo of your schematic sketchs as they mature then others can easily highlight where you may have incorrect aspects.
Ciao, Tim
I like how they reversed the sex of that "SUPPLY" connector, between your OP photo and the above picture - Yikes! I wonder what it actually connects to?
Many moons ago I suggested cascading all those gain stages (after all, for guitar you only need one input) with an appropriate level of resistor divider signal reduction between each. If only because they're available to do so. Using a trim pot to reduce the level into the next stage would allow finding, perhaps, a sweet spot where stages 2, 3 and 4 contribute nicely to the distortion generated by such a cascaded gain stage topology.
I imagine a 3 - 4 gang log taper volume control would be a bit difficult to find these days. I think I had one - once. Idea stolen from a Bogen amp I saw - again, all those years ago - which had a volume control having more then a single section.
gram inputs probably already close to 1 meg input.
But have a lot of gain
4 channels and 2 band tone control.
Looks ready to go.
clean sockets, retention
re cap and maybe slight adjustment to input
jacks. 15 and 7.5 ohm taps. 16 and 8 good enough.
plenty of drifting resistors, figure out the bias.
See how the output tubes are done.
With typical EL34 usually screen resistors added
if you are gonna clip it or run it hard.
But have a lot of gain
4 channels and 2 band tone control.
Looks ready to go.
clean sockets, retention
re cap and maybe slight adjustment to input
jacks. 15 and 7.5 ohm taps. 16 and 8 good enough.
plenty of drifting resistors, figure out the bias.
See how the output tubes are done.
With typical EL34 usually screen resistors added
if you are gonna clip it or run it hard.
That amp appears to have four addition pots retrofitted? Every guitarist is different, so one way to appease is include a pot after every stage if practical, and let the player overload different stages to suit what they like (if they don't mind some 'tone').