Hum issue in 18 watt 6bq5 build

[Danelectron]

I just finished building an 18 watt type amp with 2-6bq5 tubes from this schematic: http://www.dreamtone.org/images/18wattLayoutactual2.jpg .

I altered it to use a 5Y3 rectifier. The power transformer is an Allen TP-25 with a TO-22 output transformer. It has a higher than normal hum and I'm wondering about the ground paths on the referenced schematic. I broke the path between the tone pot and the chassis ground, but that didn't change the hum level any. Allen claims their center tap for the 6.3v heater winding makes a 100R pseudo-tap unneccessary. Would I be better with an artificial tap or would I want to break the ground path from the filter caps and the preamp section? Looking at the schematic, it looks like a big ground loop, but I don't want to just start chopping it up if I don't have to.

Thanks for any input, Dan

[olddawg]

I have found in 18 watt clones lead dress is very important. I gotten a couple of builds as basket cases people have given up on. Tightly twisted filament runs located along the edges of the chassis helps a lot. Also remember the right angle rule. PT orientation matters as well. It's a lot of small details. I have found the Ceriatone layout arrangement to be very reliable. I would think a center tap would be less problematic than a virtual CT.

[loudthud]

Quick test: Hook the amp up to a speaker but with nothing connected to the guitar input. Turn the amp on, let it warm up and put the Standby switch in the Play position so you can hear the hum. Now flick the Standy switch off. Does the hum die immediately or does it fade out over a period of a few seconds? The hum that fades out is from the heater wiring. The hum that stops immediately is from bad grounding.

I looked at the layout you linked above and it is not good as far as grounding goes. Read the links in the visitor messages section of my profile for discussion of how to rewire the ground.

[Danelectron]

olddawg, This amp is set up similar to the ceriatone TMB layout. I did set up a virtual tap for reference and the hum was no better. In fact it may have increased a bit so I am going to reverse that. The heater wiring is twisted well and isolated on the opposite side of the tubes from the board so should be ok.

loudthud, Thanks for the tip. The hum shuts down immediately. With no input I also noticed that if I turn the tone all the way to one direction I get a high pitched squeal. That squeal fades out slowly when the standby is flipped off. I didn't get the squeal before I set up the virtual tap so I'm going to reverse that. I have built several other amps, some with star type grounding and some with buss type grounding, but I have always kept the input grounds seperate from the filtering cap grounds. I will make some adjustments. That layout looked bad from the start, but I kind of assumed it was a tried and true design. I mean, it IS on the web.(sarcasm)

Could mismatched tubes cause this type of problem? I stuck in a couple of old tubes that I had on the bench. Both are RCAs, but one is a black plate and one a greyplate. I don't have a tester, but did notice one glows brighter than the other. The voltages are running a bit high, but not too bad. I'm going to order some tubes, adjust the voltage divider a bit and redo the grounds. I'll post back results. Thanks for the help.

[Danelectron]

Problem solved. I bought new tubes and I seperated the grounds between the first and second filter caps and tied the down stream caps to the buss ground. It is extremely quiet. The voltages are running about 20 volts high so I will have to address that. It sounds great though. It has lots of headroom and breakup starts at around 6 and really gets gritty from there.