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Old 1st May 2013, 07:40 PM   #1
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Default Options for how to use this old tube PA amp? > 200W

As part of my experimenting and learning about tubes and circuits, I picked up this 1960's vintage tube 70V PA amp system last year. Made by Schulmerich for use with carillons in church bell towers. It consists of three very heavy chassis- driver, power amp and power amp psu.

After putting some time and money into it, I've got it working. However, I subsequently discovered the design has a fairly steep high end rolloff so it is not a good choice for hifi, which I had naively hoped to use it for And the OPT needs a 32 ohm load, not generally useful without a line matching transformer (as it was designed to use).

I've had fun and learned a lot, but now that I've repaired it I don't know what to do with it after I discovered I won't want it for hifi. Suggestions?

Limitations: due to size and weight it is not very portable, so I am not sure a guitarist would want it. The 805 output tubes are expensive to replace. The three pieces would take up almost a 21U rack, weigh about 150 pounds, and I would need to custom make rack ears for it. A line matching transformer costs about $70. By the time I pay for line matching transformer I will have around $400 invested in it- which is more than most guitarists would want to pay, I think. It would be great as a permanent PA system, but modern systems are cheaper, have lots more features, and easier to service. Replacing the OPT would by expensive, technically involved, and time consuming. The only real advantages of it are the power and the tube sound quality (I hope- since I don't yet have a matching LT I haven't played these through a speaker yet). Also too heavy for me to ship, so I could only sell it if the buyer is local.

I just thought maybe some of you could give me some perspective on what this might best be used for, other than a paper weight, since I don't know much about tube amps or the tube amp market. I can post photos or schematics if people show an interest.
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Old 1st May 2013, 08:14 PM   #2
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It can be possibly be fairly easy to rebuild it as a hifi amp. 32 ohm speakers are not that challenging to make if you are into DIY.

The most important thing to find out is if the HF rolloff is in the circuit design or in the OPT design. Can you post the schematic?
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Old 1st May 2013, 08:17 PM   #3
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yes, pix, schematics!!

805 tubes, a pair? Or more??

a pair of 16 ohm speakers in series would do fine... heh. but massive power there...

You have a pair of these, or just one??

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Old 1st May 2013, 08:17 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

It would make a presumably nice sounding mono vocal PA amplifier.

It wouldn't be difficult to knock up a pair of 16 ohm PA
speakers and use it to drive the pair connected in series.

Guitarists or Bassists 4x12 8ohm cabs can be wired 32ohm as well.

rgds, sreten.
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Old 1st May 2013, 08:59 PM   #5
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Power amp schematic (by inspection):

Click the image to open in full size.

Yes, I have two of them, that is two power amps and two psu's, but the driver amp chassis contains both driver channels.

Click the image to open in full size.
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Old 1st May 2013, 09:05 PM   #6
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I should say- I have had several people helping me try to figure out if the high freq rolloff is due to the OPT or some other design place, but no definite answers. I assume, and correct me if I'm wrong, but replacing the OPT would be expensive and possibly difficult. For instance, I don't know what properties the 5&6 tap on the primary actually do, or what if any interaction there is between it and the other taps. Also, that's 1200V feeding it, not sure how common that is in available OPT's.
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Old 1st May 2013, 09:15 PM   #7
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Oh, and here's the driver amp OPT turns ratios:

pin2 (green) 80
pin3 (green/yellow) 44
pin4 (grey) 21
pin5 (white) 7.7
pin6 (red) 3.5

With an assumed power amp push-pull output impedance of about 5800 ohms, and the power amp OPT turns ratio of about 13.5, that gives an expected 32 ohms output impedance. I've been using the red driver OPT tap, but the white gives similar freq results.

If anyone has specific OPT replacement options I'd like to hear them (250W).
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Old 1st May 2013, 09:59 PM   #8
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If it's meant to dirve a 70v line, it might prefer even more than 32 ohms. I don't know anything about an 805...what are its specs? It looks to me like it has potential to be a very cool bass guitar amp, with a new output transformer. Post up some pictures, with a hand or ruler in the pic so we get some idea of scale and how thick the core laminations are in the transformers.

In an ideal world, what impedance do you want the OT primary to present?
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Old 1st May 2013, 10:12 PM   #9
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Quote:
Originally Posted by cyclecamper View Post
If it's meant to drive a 70v line, it might prefer even more than 32 ohms.
What makes you say that? 70V and 200W gives 24,5 ohm on my calculator...
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Old 1st May 2013, 10:36 PM   #10
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the input signal, does it really go through a separate winding on the OT?? (5 & 6) Seems wrong to me. Right off the top, I'd try feeding the input iron directly, remove the connection to the OT. Watch out though, this may be negative feedback. Unclear how that works with the input signal going there...

I'd also test that winding and try to see the DCR and the turns ratio.
(a 10vac nominal transformer as a signal source will a nice job).

Also how do you know that is a 32 ohm secondary?

The freq response of the input iron can be determined separately... no tubes in it... it already has a load.

The output iron (drive the input iron directly) can be tested as well... look for a bump in the response before it rolls off... that usually shows the max usable frequency. Use a 32 ohm resistor if that is the nominal value.

Of course check the driving amp, that may be where the frequency restriction or contouring is going on... although nothing jumps out in the schematic... see what the driver amp does for freq response... except for the .001 + 20k, but that may well be above audio freqs...

The 1200v is not a problem for OTs designed for the bigger vacuum tubes...

But quite frankly, you may make out far better IF you want an amp like this by selling this one working and using the proceeds to buy the parts to assemble a purpose made pair of large vacuum tube power amps - be they 805s in PP or not. There are a ton of possibilities.

_-_-

Can we get a jpeg of the units?
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