Reeves Custom 100 (Hiwatt DR103) Issue

[Corry]

So I have a friends Reeves Custom 100 head which is a clone of the DR103. The issues he was experiencing was an interimittant static sound and output tube failure. I found a bad output tube, blown mains fuse, and some noisy carbon comps in the PI circuit. I replace all that and install/bias a new set of tubes. Also cleaned all tube sockets. I notice a high pitched “splattering” distortion with a lot of sag/compression when the master volume is anywhere past 2 o’clock. The issue is exacerbated by the channel volumes being cranked and by the bass/treble controls being turned up. Especially bass. This was all when using the 8 ohm speaker tap. When on 4 or 16 ohms with the corresponding speaker load, the issue isn’t quite as prevalent but with the volume high and bass set high the same issue is there. But it’s the most obvious on 8 ohms. When playing loud the mains fuse blew again and an output tube went bad. Found cracks in 3 of the 4 output tube sockets between multiple pins and arching on one of the sockets(one that went bad). Replaced all the tube sockets with the same persisting issue. Output circuit wiring was done following any of the old Hiwatt DR103 schematics available online. Contacted Reeves and they said the amp appears to be a one-off and suggested I wire the output section to mirror their current production Custom 100 amps which is the same as the Ceriatone layout except for one small difference in the bias circuit and a small change on V3 preamp tube. So all output tube sockets are new, used new 1k 5w resistors, 22k grid stopper resistors, and a new 470 ohm 10 watt HT resistor. After rewiring the output section the issue is stil there. Have tried another set of output tubes with the same result. Have tried other preamp tubes and have tried a 12AT7 in the PI but still getting distortion on low notes that sounds like frying bacon. Voltages appear to all be fine. Amp is biased to -38v. Tried strapping an electrolytic across the filter caps to see if that helped and didn’t notice any difference. Have went through the amp with freeze spray without any effect. Tried adding a 47pf cap on the PI outputs but that didn’t help. The 470 ohm HT resistor previously installed was smoking when the amp was being played at full volume. Replaced that along with everything else and it seems to get overly hot. Output tubes don’t redplate or appear abnormal at all but they’re getting quite hot. I attached an audio clip of the issue. You can hear it after the initial attack of the notes. Also, it doesn’t do it in the first few seconds of being on and cranked but after a few seconds of playing it becomes prevalent. And I notice a “phaser” type of effect going on when it’s cranked. Seems to be blocking distortion to me but I can’t find the cause. Ideas?

http://www.ceriatone.com/ceriatone/wp-c ... hat103.jpg

[martin manning]

I’d be looking at the output transformer. Low voltage testing may not show a fault, but the symptoms you describe could be caused by a failing OT.

[Stevem]

It would not be the first time that not too old power supply filters have developed high ESR, and yes failed output tube sockets may have done harm to the OTs primary side.

[Corry]

A bad OT is what I was fearing but it’s the only variable left besides the filter caps. Is there a reliable way to test ESR without buying a capacitor tester? For the OT I will probably just source one and tag it into the circuit to see what happens. It’s crazy how the simplest of amps seems to be the worst at troubleshooting. Only had two other instances of bad OT’s and one was dead and the other was just very weak in volume. Never had one mostly work. Thanks guys!

[Stevem]

There are 2 cheap ways to tell if a OT is bad.
One way that cost you nothing is to look at the output tubes in a dark room when the amp is cranked up.
The inside of the tubes will glow brighter when your playing and the tubes are driving current into the OT.
If you can look at this effect in a normal ruuning amp so you can judge better what going on .
If the inside is kicking up brighter yet the amp has low volume then it's very likley new OT time.

The second way to tell involves installing a 1 ohm 1% 3 watt resistor from each output tubes cathode to ground.
Doing this and then hooking up a D.C. Voltmeter across the resistor will allow you to directly read how much current the tube is pulling from idle on up to the max it can yank out of the power supply.
Once again if you play the amp cranked up and you are reading over .090 volts on the meter which is really ,090 amp but yet the amp has weak output , then yes this would confirm a OT with a shorted winding!

[Corry]

There are 2 cheap ways to tell if a OT is bad.
One way that cost you nothing is to look at the output tubes in a dark room when the amp is cranked up.
The inside of the tubes will glow brighter when your playing and the tubes are driving current into the OT.
If you can look at this effect in a normal ruuning amp so you can judge better what going on .
If the inside is kicking up brighter yet the amp has low volume then it's very likley new OT time.

The second way to tell involves installing a 1 ohm 1% 3 watt resistor from each output tubes cathode to ground.
Doing this and then hooking up a D.C. Voltmeter across the resistor will allow you to directly read how much current the tube is pulling from idle on up to the max it can yank out of the power supply.
Once again if you play the amp cranked up and you are reading over .090 volts on the meter which is really ,090 amp but yet the amp has weak output , then yes this would confirm a OT with a shorted winding!

Great info! I will try both these things. There are already 1 ohm 3 watt resistors on the cathodes to ground so that will be easy enough. I did have a voltmeter attached while playing but I didn’t pay a lot of attention to how high it went after I biased the tubes. I know it was at least over .080 amps. I’ll do these things and see what happens. Thanks dude!