Calling All techs!hot rod deluxe : hum&red hot plate,6L6

[tonequester]

Greetings.

My brother-in-law's hot rod deluxe had hum problem. HE called me to check it out. Amp starts up with no noise, then gradual increasing hum. Cleaned and tightened chassis grounds. No good. Took chassis out for better look see. Noticed one 6L6 plate increasing glow to red hot, as hum got louder. THEN HE told me HE had re-tubed with Grove Tubes a few months ago, and re-biased using bias test voltage method. NOW I'M WORRIED ! Checked test point and found ZERO voltage ! Before shutting it down, there was also distortion and slight motorboating while playing guitar. HE says all was well BEFORE he replaced speaker last night. I haven't swapped 6L6's yet, but don't think it could be a bad tube.
Visual inspection shows NO burned spots anywhere. New Eminence speaker(8 Ohm) seems O.K., tested resistance at 6.8 Ohms. I have some experience at repair and modding tube amps, but I haven't come across this before. I have reference books that I will check, but he has a gig to play end of this week. I suspect a short somewhere.
If anybody has knowledge or advice to share, I would be forever grateful(and HE should too). PLEASE help ! tonequester.

P.S. I apologize, as I don't know how to post a schematic, although I found one.

[Stevem]

Is only one 6l6 red plaing, if so swap tubes and see if it follows the tube which would indicate a bad tube.
If not what is the bias volatge on pin 5 of each tube, it should be well over -40?
Since he shoved in new tubes, check the solder connections on the sockets for rings aorund them/signs of a bad connection.
Make sure that the ribbon cables are not snapped off at a end that jump from board to board..
No short problem or the amp would be blowing fuses.

[tonequester]

Greetings stevem.

Thanks for all of your info on the Hot Rod Deluxe problem. My brother-in-law evidently dodged a bullit. After getting it over to my place, on my work bench, I pulled the tubes and used contact cleaner on the tube sockets. I can't believe it, but I hooked up the speaker and my guitar, and the damned thing worked to perfection. However, although I did have -47 Volts at pin 5, both tubes, I only have 72 mv at the bias test point. If I am remembering correctly, my brother-in-law told me that the bias was supposed to be 90mv. I guess I'll leave it up to him whether to fool around with adjusting that, or not ! Better to run too cold than too hot reliability wise, although the tone may suffer. He is all country, so he likes it clean. I'm going to try to look up that bias Voltage in the meantime, and leave it up to him. He needs to dump the amp soon though. the power transformer is rust city !

THANKS AGAIN for your helpful reply !!! Best Regards, tonequester.

[Teleguy61]

Glad your problem seems to have cleared up easily, but you will need to keep an eye on the amp.
The biggest issue with the HRDX and its kin in the Fender line is failing solder connections around the output tubes, due to poor solder connections on the pcbs. They go intermittent and sometimes fail, due to vibration or heating or both.
A serious design flaw.

[Stevem]

36 ma for each tube (72 total) is plenty ( assuming the tubes are matched) and should be great for country pickin!
The PTs in these amps are on the small side mass wise and as such having the B+ working at 8% less should keep the PT that much cooler also.

[tonequester]

Greetings stevem.

I'm glad to know that the bias is good to go for my brother-in-law's gig this weekend. It's a darned shame that he let that amp set around in a barn when not being used. It has drawn enough moisture that the PT is very rusty, no paint left. I told him he needs to dump it a.s.a.p., before he gets trouble with Eddy currents or flat out PT failure due to the rust. Oh well, Now he'll have the chance to get something a little less powerful. He is always complaining about blowing everyone away with that amp, and you don't need 40 Watts when you only play SMALL indoor gigs, like he does. He plays one so small, that he could get by with my Blackstar 1Watt practice amp !

Hey, thanks again, for ALL of the advice. I appreciate it, and my brother-in-law does as well ! Best Wishes, tonequester.

[Stevem]

That eddy current thing is a ton of BULL CRAP!
Murcury Magnetics did a test with two OTs and left one outside unprotected for a year.
when the tested the two out the one that was left outside worked a tad better and ran cooler!
If he can only store it in a barn just stuff it in a big plastic garage bag like for lawn use.
Have him pick up one of the Weber attenuators for under a 100 bucks to get the volume down, its still cheaper than a new 1 watt amp.

[pops]

If he likes clean and wants to quiet the amp take all the cathode caps off the preamp tubes and replace the preamp tubes with 12AY7's. will quiet it down some and give a flatter response.

[beasleybodyshop]

Glad your problem seems to have cleared up easily, but you will need to keep an eye on the amp.
The biggest issue with the HRDX and its kin in the Fender line is failing solder connections around the output tubes, due to poor solder connections on the pcbs. They go intermittent and sometimes fail, due to vibration or heating or both.
A serious design flaw.

THIS - I have had several HRDX on my bench in the last 4 months, and like clockwork the issue was bad solder joints on the power/preamp tube sockets and ribbon cables.

[tonequester]

Greetings Guy's !

Hey, Stevem, I wonder if the PT left outside actually rusted. This PT hasn't got a bit of paint left on it, and the rust is definitely penetrating between leaves. I wasn't talking from experience earlier, just from what I've read. I keep MY gear INSIDE and DRY. Thanks pops, for the advice on the cathode caps and 12au7's for taming that beast. I'll run it by my brother in law. The only problem is that 6 months ago he popped big bucks for some overpriced Groove Tubes, even the pre-amp tubes, but the Weber attenuator is a good idea. I'm getting ready to get a Mini-Mass 25W. myself, with a headphone tap. Their speakers are great, in my opinion. Hey, beasleybodyshop. I appreciate the "head's up" on the bad solder connections on the pcb. I'll let him know........well, I'll be the one to check it out, IF anything else happens ! Tubes and pcb combinations suck ! I'll stick with handwired on eyelet board myself. I don't need that much power either, and I like to do a little modding anyway. I'll stick with my simple VHT. if I need more power, I'll build a s.s. power amp from a kit for less than $100.00 to feed the attenuated VHT to. Oh, who am I kiddin'. I won't be playing any gymnasiums soon ! Thanks guy's....for the replies. tonequester, Best Regards !

[pops]

You most likely won't like the au7's, but ay7's will give a much better sound. If you want use one au7 and move it around and see if you like it, but i rarely use anything EVER only 12ay7's. ax7's are too hot and bright for my tastes. You will like the no cap and you can put it on a switch and have it or not, or use a on off on and have one size cap, no cap or another tonal change by a completely different cap size. have fun.

[EtherealWidow]

^Ha. That's funny. I thought I was the only weirdo that did that. I like to use 12AY7's in my high gainer as well. Smooths it out a ton and gives a lot more definition.

[M Fowler]

Yup run 12AY7 in my hot rod plexi too hot with 12AX7

[pops]

I feel like i am not alone now:)

[Stevem]

It was rusted like a cast iron water main under NYC.