peculiar PA tube amp conversion

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Hey there,
First off I should say my knowledge in electronics and particularly in tube amps is very limited and I'm not intending to put my life in danger and mess around with it, but mostly wanna know what scale of an operation is needed for this amp to be turned to what I want it to be.

Here's the story - It's a tube amp made in 1969 by an Israeli long defunct brand called Halilit. From what I understand they made pretty solid Marshall Clones and never had their own designs. This head as you can see in the pictures has 4 inputs with 2 jacks for each input (I imagine they are low/high gain). When plugging in a guitar or a bass to input 1 or 2 I get a really sweet sound, but the volume doesn't get anywhere near what an amp with transformers this size usually puts out. Even when its turned all the way up the volume is still pretty moderate, however its super clean all the way up. This leads me to to think those are line level inputs.
Inputs 3 and 4 while plugging in a guitar or a bass give louder, but distorted sound even in low volumes, and my assumption is that they are phono inputs.

My question - what does it take to make one (or more) of these inputs suitable for a guitar or a bass so I can utilize all the potential that this amp has? Is it a matter of replacing a few components in there or is it more of rebuilding a whole section of the amp so that it has the right impedance for a guitar?

- I realize that if I had a schematic it would have been a lot easier to help me solve this mystery, but unfortunately the shop that made this amp doesn't exist and I have no way to know what would be the Marshall equivalent of this head.
- I know it says on the front panel 200W, but from what I understand its just a label and not meant to be the right wattage of this unit.
- I'm attaching a few pics of the outside as well as the guts of the input section and hoping this could help to determine what kind of input those are, though if this is completely useless please do tell.

Thanks a lot,
Ayal
 

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Hey,
Thanks for the quick response.
I've actually plugged my bass once through an audio interface that's supposedly puts out a line level and the volume I've gotten from the amp was much louder.
I'd prefer to be able to get this kind of volume without more stuff in the chain if it's not too complicated to achieve.
 
There is a master tone section, then each of four seemingly identical input channels has a tone control and a volume. If I had to guess, I;d say you have an old four channel PA head, not a guitar head. But I could be wrong.

Halilit still exists in Israel, and it couldn't hurt to contact them and ask if they still have any information about this product.

Do you have rear panel photo and photo of top side of chassis?
 
I know this is in fact a PA head.
The thing that bugs me is that most PA heads have inputs that are designed to take a signal from a mic. This doesn't seem to be the case with this amp and moreover, the 4 inputs don't seem to behave in an identical way.

Theoretically if they were mic inputs I could have plugged in a bass through a DI box and get it full potential of it as far as volume goes, but since this is not the case I'm trying to find out how complicated would be a modification that would allow me to use it for a guitar or a bass comfortably and without any additions such as a third party preamp.

Here are some more pictures of the amp, I hope this is what you were looking for.
 

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I saw you reported the channels did not act the same, but that may be an issue with problems in the amp.

FOur EL34s suggests it is a 100 watt head.

Considering its age, you may have aging caps causing trouble. It looks like a lot of parts, but the power amp is no doubt utterly conventional, looks like the last small tube is the phase inverter, leaving three dual triodes - I assume - for the four channels. Just a guess from looking, but maybe two tubes do the four input stages, with a shared cathode in each tube (note the cathode resistors are 820 ohm instead of the expected 1500 ohm were there separate resistors). On the left I see only two cathode bypass caps with resistor, but four 100k plate loads with coupling caps. The coupling caps feed the two left most volume controls, and it looks like the volume control wires from the other two slant back over to the same area. That leaves one more dual triode for a gain stage and maybe a cathode follower for the tone controls.

Point being that I can almost draw out a schematic from here, and could if we had a better view of the tube sockets themselves. You could fairly easily draw up a schematic from the chassis.

As to the problem (or not) look at the left and second left 12AX7 (tell me if they are some other type tube). Pins 1 and 6 are plates, do all four have about the same voltage? or different? Also the two cathode bypass caps (the blue ones to the left), do they have about the same voltage on each? If the left pair reads different from the other pair, that might explain why channel 1/2 sounds different from 3/4. And make sure none of those coupling caps are leaking DC to the following circuits.

I don't recall any from that era, but conceivably it is not a mic mixer but some sort of line level mixer for say keyboards or something, as you suggested. On the other hand the signal from inexpensive Hi-Z mics is a lot hotter than low Z mics put out. And those are not low Z inputs.

PAs were never meant to be driven into distortion, but considering all those channels and their tubes, once you determine it to be all working, you could then repurpose a couple of the channel as extra gain stages for the first two channels. Even arrange some switching. No reason this can;t become a screaming head no one else has.

Maybe a LITTLE like this:
http://bmamps.com/Schematics/Sunn/Sunn_Studio_PA_Schematic.pdf

With just a touch of this, minus the reverb:
http://bmamps.com/Schematics/fender/PA100_Schematic.pdf
 
Hi!
Enzo is spot on... There should be more than enough gain in that amp's setup for a guitar input. Since it's a PA amp, it will not breakup much, if at all. Now this doesn't mean that you can't rearrange a few components in the input stages to turn this into a guitar amp with great tone and breakup.

Your amp is currently wired something like this:

http://www.drtube.com/schematics/marshall/1963-70.gif

What you want is something wired like this:

http://www.drtube.com/schematics/marshall/1959u.gif

As you can see, there isn't much difference between the two.

As to why, your amp has low output... I would get all the tubes tested to make sure they are up to spec. I would also check the voltages at each filter capacitor node to make sure the voltages are correct. The 2 biggest things that go bad in amps this old are the tubes and filter caps. One more thing... The carbon comp resistors can also get out of spec because of age and environment; make sure they are all in spec.

The way the amp is set up, It's a 100 watt amp. If those other chassis 2 holes were filled with EL34s, for a total of 6 EL34s, then it would be a 200 watt amp. It's a nice amp... With a little work, you can have a great sounding guitar/bass amp. You can make each of the 4 channels configured differently; IE. One for bass, one for plexi style guitar sound, one for JCM800 sound, etc.
 
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