Peavey delta blues

[redmojosteve]

There seem to be afew of these threads so I apologise by starting a new one but I haven't seen this issue before. The amp is my son's Peavey Delta blues and it worked fine until it started one of the EL84's started to get hotter and sound quite microphonic. It's the one which is second from right looking from the rear of the combo. Before long it started popping and then failed. The valves were replaced and all was well for a while after which the cycle of self destruction repeated itself. I opened it up and unravelled the 3 PCB's, repaired a burnt out track (!) which was in the pre-amp section, replaced a blown fuse and no improvement. Incidentally I had put another valve in the above location and it was very microphonic whereas the others were not. The offending valve position appears to have a high heater voltage at about 20V but I have only checked this without the valves in place, but having looked at the schematic this would not appear to give me a good reading anyway. I have replaced the diodes in the power supplies and the protection diodes (GP1250) have tested as OK. None of the resistors look fried. Any clues would be appreciated.
Steve

[Amp Kat]

There are 2 diodes coming off of the plates of each set of two EL-84's CR4 & CR6. Make sure they are ok. You should get a bias rite so you can measure the currect thru each tube and see if they are all biasing up the same. The heaters are ran in series for the power tubes from a 25.2 VAC and 6.3 volts is dropped across each tube. If one tube is pulled , they all go out. This 25.2 volts is then rectified and filtered into 36 VDC and the 3 preamp tubes are ran in series where 12.6 is dropped across each tube and the same as the power tubes apply as far as pulling one out kills them all.

[redmojosteve]

Thanks for the quick response. I have tested CR4 and CR6 out of circuit and they appear OK. However I was planning to replace them just in case. I'll do a detail check of the components around the specific valve (sorry tube!). I think a bias rite would be a sound investment and can still afford one even after today's depressing budget in the UK. Finding a UK supplier is another matter..
Thanks for a great forum. I spend quite a bit of time on the DIY Prodigy forum and it's good to see these virtual venues for support and ideas.

[Enzo]

The bias supply in the amp is not adjustable. A Bias Right won't be much use in that case. Bias is not the issue here. Well, other than a loss of it or something.

[Amp Kat]

I should have specified that more clearly as the Compu-Bias Meter from Mission Amps which reads plate voltage, current and wattage dissipated. He could see if the tube was actually working or just the heaters were lighting and the tube was null.

http://www.missionamps.com/

[Enzo]

Oh, you are right, it is a good tool to have, and helpful in diagnostics, I was just thinking of the immediate issue. If the tube is red plating, we know current is off the chart.

[redmojosteve]

Thanks Amp Kat. I replaced the diodes CR4 and CR6 coming off the plates and all now seems well.