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Options for how to use this old tube PA amp? > 200W

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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 :rain: 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.
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?

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.
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.
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).
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?
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? :D
Oh, 805s are class B zero bias tubes.

They can be used for lower power class A amps - the chinese are making some now...

In PP class B (AB1) they'll do 200watts loafing. AB2 is a better bet if ur building a serious amp.

These are not likely to be useful for bass amps... certainly not bass guitar amps, imo!

Saw the turns ratio stuff after I posted... fyi.
That OPT has lots of taps. I bet green is 4 ohms and green/yellow is 8 ohms.

Close. My calculations say the green one presents about a 1 ohm impedance, and the green/yellow presents about 3 ohms.
I think the 0.001uF and 47K are rolling off your highs. Just clip the circuit in a convenient place to check.
There's been some discussion about that on another forum. Seems the general consensus is that the cap is an essential part of the feedback system and "shouldn't" be messed with. The 47K resistor is part of feedback for that tap only- but the other taps show the rolloff as well.

I only have "before" photos right now (with no tubes), here's a few. I will post some newer "after" photos soon. The driver amp contains two completely independent channels.

@Bear- yes, the signal really does go through that winding (pins 5 & 6), evidently some kind of feedback.

I checked tonight (finally) and a signal put into the power amp directly (bypassing the driver amp) still shows high freq rolloff.

I think if I could replace the OPT with a better audio grade one for the same price (?) as a matching line transformer I'd rather do that.

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The 805 is in the same "50 watter" class of valves with the type 211/VT-4-C and the 845. By adding appropriate bias, either of these can be plugged right into the 805 sockets, including the existing (and hard to replace) output transformers.

One option you might then consider is to remove the driver amp's output transformer and the final amp's input transformer. They're not really needed for home use without grid current, and are bound to be limiting your bandwidth some.

All good fortune,

It's ur stuff... do as you wish.
My best advice is to try to sell this setup (cleaned up a little would be a good idea) and turn it into something more suitable for your use.

When you say replace the output for the same price as a matching line transformer, do you mean a step down external transformer to match down to 8 ohms?? I'd advise against that. Regardless once you buy a new OT, you might as well just build up a new amp - the chinese are selling beautiful ready made chassis these days for not much money. Add a few PS transformers and ur most of the way there.

I really suggest running the input direct to the input transformer, with the pins 5 &6 connections not present, to see if that helps with the frequency response.

Where exactly is it rolling off??
How are you measuring?

What is providing the signal?
You do not need to use the original unit to send signal to the input transformer - you could use a solid state receiver if you wanted to... be sure of a flat freq response, etc... but loose the pin 5&6 connection for a test...

Yes, I agree- if I can sell it I will. But as I've said, I don't see a market for it, unless I find a musician type who is eager to load it with a 32 ohm speaker setup. And who doesn't mind the rolloff. Maybe I'll be lucky with that.

The 32 ohms was derived from measuring the turns ratio of the OPT (13.5), squared and divided into the estimated output impedance of the 805's- about 6K. 6000/(13.5^2) = ~ 32.

One thing I've noticed... if I reduce the load (higher resistance) the rolloff flattens out. Using a 200 ohm resistor as the load the response was acceptable, higher would be better. Could I get an hifi grade transformer to load this OPT to 200 ohms or so when connected to an 8 ohm speaker? That would be, what, sqrt(200/8)= turns ratio of 5? I guess that would decrease the power, but by how much?

But Bear says "I'd advise against that". Why?

To answer Bear's question, I have measured the response with a signal generator, a scope, and a Fluke lab-grade RMS DMM. For instance, with 1KHz input to the driver amp, output was 3 VAC, while at 20Khz it was about half that (30mV input).

I'm investigating the use of that pin5&6 winding, but I don't hold much hope for using the circuit without it. and I don't have the skill or interest in modifying the circuit too extremely. I agree with you- I'd rather sell it if I can't figure a simple way to flatten out the response.
Nothing to modify, just drive the amp with another amp. Not the original driver amp. That's first.

Then drive the amp with the pins 5&6 desoldered, or rig a temporary RCA jack to the input transformer with the wire to pin 6 disconnected. Drive it that way. Skip whatever the pin 5&6 are doing. See what it does WRT frequency response. Drive it with your Crown amp... skip the driver amp. You want to see what the native response of the 805 amp is.

WRT finding a 200 watt "matching transformer". It is possible, but the issue is going to be both frequency response and expense.

When you make the secondary load higher, it is reflected back to the primary, making the load there appear higher in Z... not hyper critical, but perhaps not optimal. Also 6kohm seems in my less than lucid recollection to be too low for 805s in PP... about half of what they want. Fwiw.

I think you may be underestimating the collector value of these amps... there was one with the rack on epay that had a buy it now for several thousand... not sure if it sold or not, but nothing much to lose by trying...

Eliminating the driver amp and using a small Crown amp instead (pre-tested to confirm it has flat freq response) to drive the power amp I get these figures:

flat up to 2KHz @ .62VAC
10KHz -> .55VAC
15KHz -> .49VAC
20KHz -> .44VAC

So, evidently something in the power amp is producing some of the rolloff. So, I think, might as well try Bear's idea- unsolder pins 5&6 then tie them together and run the test again. The purpose is to bypass that winding in the OPT to see if it is designed specifically to reduce the bandwidth.

I happened to be monitoring the HV going into the power amp as well as the in and out waveforms. Normally the HV is stable at around 1110VDC. When I powered this test on the HV fluctuated around 300VDC. So I immediately powered down. Thankfully no harm was done- those 805's cost $100 each NOS.

I think I'll do no more messing around with the circuit.

The original purpose of this thread was to see if anyone could suggest a good use for this system knowing it has a high freq rolloff. Evidently (from hearsay) the horns used with these type systems may have been rolled off at 15KHz by design, though my testing shows the rolloff with this system starting at about 10KHz.
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You may be making more of the HF roll off than is necessarily going to be audible in use, there are plenty of SE amplifiers that do comparably in the HF, you are about -3dB at 20kHz which in use may prove perfectly acceptable, and given the available headroom perhaps a wee amount of EQ might be acceptable in the driver to flatten things out.

Bear may have a point about their value.

And you can wind an autotransformer or get edcor to do it for you with an 8 ohm driver a ~2:1 stepdown would be fairly easy.
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