Valve PA to guitar amp

[peacocksuit]

I have an old British PA amplifier which I am hoping to use as a guitar amp. The valve line up is EL34, EL34, ECC81, 6BR7, 6BR7, ECC82. There are also 2 EZ81 rectifiers.
It has 4, 8 or 16 ohm taps on the OT. It hasn't been used in decades and never by myself. I had originally planned to replace the old capacitors and then see how it worked as a guitar amp 'as it is'. However, given the amount of time and effort it would likely take to get to that stage, I've decided now to look for a classic amplifier schematic with EL34 power stage and strip it and rewire to replicate.  As I have MT and OT already and all the tubes are present (hopefully working but I can always replace if necessary), it seems like I have the main ingredients for a nice amp.
I'd like some ideas about which amps from the likes of Hiwatt or Marshall etc I might be able to replicate using as much of what I already have as possible. My understanding is that in push pull it should be good for around 50w, which is more than enough for my requirements.

I will add some pics shortly, I have posted on another forum already but thought I might be able to get some ideas over here 



 

[kagliostro]

First thing, if known a Brand and a Model will help

Second (a bit more difficult) a schematic of the amp (not always present obviously)

Third at least measure the Voltage levels on the amp (over all B+ level and, may be, AC HV level)

Usually it will be better to maintain the Power Transformer + The Output Transformer + The Power Tubes arrangement (as it is optimized for B+ & the OT) + the Phase Inverter arrangement

If you are able to draw a reverse engeenered schematic the first interest will be to draw the PI & Power Tube & Power Supply sections

this will give info that are important to plan a mod

Franco

[nandrewjackson]

Sounds interesting

[AlNewman]

I think your 1st plan is a good one.  Replace the main filter caps.  Also look into the input, and put in a fender style configuration with a 68k grid stopper and 1M grid leak.  That should give the best indication of how that circuit works as a guitar amp.

[acheld]

What Franco said, and

Fourth  pictures are really helpful -- and make it fun.

[ShoemanGB]

I think your 1st plan is a good one.  Replace the main filter caps.  Also look into the input, and put in a fender style configuration with a 68k grid stopper and 1M grid leak.  That should give the best indication of how that circuit works as a guitar amp.

Having done a project like this myself, this advice is spot on. Check out the inputs, and convert one to Fender spec and see how it sounds.  Or for that matter, if it has 1/4" inputs try it as is first.  Mine had two mic in's and I left one as is (grid leak bias) and made the other a 34k/1M.  The grid leak gets real Black Keys sounding easily.
  WHy re-invent the circuit if you may not have to.

[peacocksuit]

Deleted

[peacocksuit]

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I've added pics but the post is showing empty for some reason?

[sluckey]

You had posted links to 11 pics on imgbb.com. I got them all. Why did you remove the links?

[peacocksuit]

I didn't, I used the insert image button to inbed them on the post bellow it . If I click 'modify' they're all still there, will the coding bookending each separate URL

[sluckey]

I didn't, I used the insert image button to inbed them on the post bellow it . If I click 'modify' they're all still there, will the coding bookending each separate URL

The forum software does not recognize "ibb.co" as a valid url. I fixed to just show links to the pics. Those pics are also too big to imbed. Takes a loooong time to fetch the image.

[kagliostro]

When you have large image to post and they can't fit on the post, use IMGUR as host, it is free and maintain images

then copy the link and put in your post as link (so the weight of the image is not a problem)

if you edit your post you can also manipulate the width of the image to reduce the shown resolution (if you do it delete the part relative to the height, it will adapt automatically)

Note that I've modified the width value (and deleted the height part) to show the image a bit less large on the thread but if you download it the full resolution is preserved
























Franco