50s Amplifier

[Bone]

Ive just won this on ebay < Vintage Valve Amplifier,Hand Built Kit,Circa 1957,Works on eBay (end time 03-Jun-10 20:19:56 BST) > originally to renovate it.

Its supposed to be a 30 watt PA amp using 807 tubes. I had one of these in the 60s used for PA in a band of which I was a member.

I'm now undecided, do I:

Renovate it to original physical condition using modern components,
or rebuild it into a new case i.e. as a head amp?

What do you think?

[chris661]

I'd say replacing all the capacitors is a must (electrolytics dry up...). After that, it's your choice.

How about re-making the case? Keep the same dimensions and material, but it'll be all shiny and new...

[Globulator]

Or find another output transformer and turn it into a stereo single ended ampifier..

[ray_moth]

Originally Posted by Globulator

Or find another output transformer and turn it into a stereo single ended ampifier..

Surely you mean "Find two SE output transformers and turn it into a stereo single ended ampifier". You won't be able to use the existing PP UL transformer for anything other than PP.

[Globulator]

Originally Posted by ray_moth

Surely you mean "Find two SE output transformers and turn it into a stereo single ended ampifier". You won't be able to use the existing PP UL transformer for anything other than PP.

I know that's the 'right' thing to do, but I suspect a 30W push-pull OPT would not saturate on a 8W SE pulling 45mA or so... or would it?

[wakibaki]

50 years old.

With the best will in the world, this is probably approaching the end of its useful life.

Renovating it may just accelerate the processes of decay, putting greater stresses on tired components. If you renovate it to the extent of replacing the transformers it won't really be the same amplifier.

Get it out every now and then, connect it up, play something through it, marvel at the fact that it works and put it away again. Don't let anybody stand too close to it while it's running.

w

[Globulator]

Originally Posted by wakibaki

Don't let anybody stand too close to it while it's running.

[ray_moth]

Originally Posted by Globulator

I know that's the 'right' thing to do, but I suspect a 30W push-pull OPT would not saturate on a 8W SE pulling 45mA or so... or would it?

45mA current (or even much less) in one direction is definitely going to cause core saturation problems with an OPT designed for PP.

A possible way around it, I suppose, could be to load the other side of the primary with the same current as the OP tube, thus simulating balanced PP conditions.

[tubelab.com]

I know that's the 'right' thing to do, but I suspect a 30W push-pull OPT would not saturate on a 8W SE pulling 45mA or so... or would it?

Probably. I got a bunch of "80VA" guitar amp P-P OPT's. I have tested these in several P-P designs. They weigh about 5 pounds each, and pass 25 watts at 25 Hz without saturation. I have had them to 150 + watts at 1KHz without fire, smoke, or saturation. They work acceptably with a 45 tube at 30 mA. It really didn't sound that great. I took one appart and restacked the lams for a gap equal to two thicknesses of masking tape and it worked much better.

[richwalters]

That sounds like a Linear Concord or sim from Leeds. One of my first workhorse amps on the strings. I found this sounded better than the version using EL34's which had a sim chassis. Both were very portable amps.
Electrolytic caps and those 20% tol carbon resistors are a must for replacement. There is one cap underneath which must be replaced as I always remembered it failing as they did those days. I believe the negative for the fixed bias came from the centre tap of the mains tranny lifted by a power resistor which eventually failed. Despite the small o/p tranny, I've known this amp to drive 18" speakers and sound good.

richy