Raytheon tube program amp

[RSRecords]

Raytheon RM10

on power up the output had significant ac voltage (~130vac). Subsided relatively quickly. I'm assuming this was from capacitors charging? Caps have been replaced and work was done by a reputable tech. I was asked to add cabling and an output pad. Everything looks right according to the schematic. Anyway to avoid this? the gain was 3/4ths of the way up when powered on but not sure it made a difference. Sounds fantastic once warmed up but I worry about someone frying something when it's wire up to a patchbay.

 

[abbey road d enfer]

Raytheon RM10

on power up the output had significant ac voltage (~130vac). Subsided relatively quickly. I'm assuming this was from capacitors charging? Caps have been replaced and work was done by a reputable tech. I was asked to add cabling and an output pad. Everything looks right according to the schematic. Anyway to avoid this? the gain was 3/4ths of the way up when powered on but not sure it made a difference. Sounds fantastic once warmed up but I worry about someone frying something when it's wire up to a patchbay.

The schemo shows no earth connection.
If you're in a 230V country, the transformer leakage explains this.
You should use a 3-prong earthed cable.

 

[RSRecords]

I'm in the US (120) and it has been properly updated with a 3 prong ac connection. The amp has been off for a while and I'm going to fire it up and take some measurements.

Another thing: Would it be necessary to load the speaker outputs? I wouldn't think so if the 500 ohm out is connected but I worry if the amp is on and has no load on the output transformer that it might damage it.

 

[emrr]

You want a permanent load across the 500-600 output since this is power tubes.  Could be as high as 1k2, see what it does to measurements. 

 

[RSRecords]

Ok thanks. I'm planning to add an output pad after the transformer anyway. Maybe I can work that into the switch.

 

[RSRecords]

This thing looks super awesome by the way...

 

[abbey road d enfer]

When a tube amp that uses global NFB is unloaded, it becomes an oscillator, generally at ultrasonic frequency and with a very high amplitude since the uloaded xfmr acts as a resonant ciruit. The voltage developed there can be higher than the B+, which usually results in frying the output tubes and arcing the output xfmr.. Since its ultrasonic, the parasitic capacitance between windings offer a low resistance path to current, taht's why you can measure a significant voltage at the secondary..

 

[RSRecords]

When a tube amp that uses global NFB is unloaded, it becomes an oscillator, generally at ultrasonic frequency and with a very high amplitude since the uloaded xfmr acts as a resonant ciruit. The voltage developed there can be higher than the B+, which usually results in frying the output tubes and arcing the output xfmr.. Since its ultrasonic, the parasitic capacitance between windings offer a low resistance path to current, taht's why you can measure a significant voltage at the secondary..

Oh wow. thanks so much. That makes perfect sense. That also explains why the gain being up created the high output voltage (oscillation). Luckily it wasn't in this state long at all. The pad should keep a ~600 ohm load on the output at all times even when not attenuating. I was getting there by I wasn't sure why!

 

[emrr]

This thing looks super awesome by the way...

Built very much like Collins Radio of the same era. 

 

[RSRecords]

So, for padding the output, I just went with a ~600 ohm H-pad using 220r as the series and 100r as the shunt. Give's roughly 20 db attenuation.
Should I be looking at a lower output impedance to play with modern gear a bit better?

 

[Bo Deadly]

That is a really nice piece.

Maybe use a U-pad instead so that the load is 600R but the output Z is only 100R.

Also note that because it's transformer out, it doesn't really need to be balanced. It could literally just be two resistors like a 470R and 100R.

 

[RSRecords]

That is a really nice piece.

Maybe use a U-pad instead so that the load is 600R but the output Z is only 100R.

Also note that because it's transformer out, it doesn't really need to be balanced. It could literally just be two resistors like a 470R and 100R.

Ok I've read that before. Why is it that it can just be a simple t pad if it's transformer out. Does it have to do with the "floating" aspect of the output transformer?

 

[Bo Deadly]

Ok I've read that before. Why is it that it can just be a simple t pad if it's transformer out. Does it have to do with the "floating" aspect of the output transformer?

The logic is very simple. The transformer secondary and any network of resistors is a two terminal device and therefore any current that goes into one terminal must come out of the other. There's simply nowhere else for the current to go. So it doesn't matter if its an H-pad, T-pad, simple L-pad or any other kind of pad, it's still a two terminal device and therefore any current going into one terminal must be matched by an equal an opposite current from the other. So yes, this is only true because it is "floating" or isolated would be a more appropriate term.

 

[emrr]

I've never had a device like this fail to play well with modern gear, I can't put anything on a list of questionable interfaces. The feedback loop makes it lower than 500 already. You can measure whatever it is easily with the 6dB test.

 

[emrr]

Oh yes - this is a monitor amp, not a program amp. Very much like the RCA BA-4.

 

[RSRecords]

Yes, monitor amp. I misremembered whilst starting the thread. I'm actually curios now about reducing the overall gain. Would It be as simple as decreasing R11?

 

[gridcurrent]

I'm actually curios now about reducing the overall gain. Would It be as simple as decreasing R11?

1. re-wire input tube as a triode.
2. instead of output pad, use a rotary switch to select transformer secondary taps, allowing you to find the sweet spot of gain vs bandwidth.
as suggested, a somewhat higher value secondary termination should prevent transformer damage but allow the output tubes to "breathe" a little easier.
3. messing with R-11 is an option but lets assume the Raytheon engineers picked the value as optimum for gain/bandwidth/stability.

 

[emrr]

Yes, monitor amp. I misremembered whilst starting the thread. I'm actually curios now about reducing the overall gain. Would It be as simple as decreasing R11?

Lose V1 completely. One of the RCA program amps had all sorts of strapping brought out to ditch a stage entirely, insert pads, insert voltage dividers, etc.

 

[RSRecords]

Bought another one of these because I liked the last one so much. However, I am in need of a suitable power transformer for this beast. The wires are starting to crumble. Someone replaced the 5u4g with some a solid state rectifier built onto an octal plug. Not sure if that cause the wire breakdown but never the less...
Out put transformer was changed too. Looks like a pretty nice UTC CVP-1. Lacks the feed back secondary so might be interested in hunting down something else there too.

Any help greatly appreciated! Here are the tubes voltages.

 

[Evertide]

If you can't find an original output-transformer , think Fairchild 670 , Sowter and others make replacements...