Hello,
I have a nasty distorted clean channel on my 110RD50. I've posted about this amp before as it had a buzzy and hummy reverb.
At practice last week it sounded horrible when I turned it on. Noisy, distorted and not very loud.
I pulled the chassis. and found that while I had replaced the caps when I bought it, I hadn't done ALL the caps. Don't know why. So I replaced the HV ones, and powered it up. It was really quiet, and just how I expected, except for the distortion on the clean channel. It's pretty nasty, and definitely not how it used to sound.
So, I've done the following:
Replaced the tubes.
Checked for +/- 8V and +/-16V on the op amp - all OK.
Checked for 32V on the zener diode - OK
Checked the switching IC.
Where else can I look? Any help really appreciated. My electronics skills are OK - I've built and repaired a few tube amps, but my troubleshooting is rubbish!
Is the distortion only there on the clean channel?
Hi Ron,
the common crossover distortion issue in these amps can sometimes be masked on the distortion channel. If you dial in a cleanish sound on the distortion channel, does it clear up or do you still have the buzz in the background?
You need a scope to really get to the bottom of the problem.
Cheers,
Lars Verholt
Thanks for your suggestion
Thanks for your suggestion Lars - I don't think it's crossover distortion. In my experience you can't usually hear COD as it occurs so close to zero volts.
This is definitely clipping, like a transistor amp turned too loud. Either the preamp is overdriving, overdriven, or ..? Does having DC on an op amp input cause this? I haven't checked for that yet.
Music Man distortion
Just in case this works for you and you haven't had any other positive results......I have an original 212HD one thirty that I purchased new in 1979....about five years ago I had some similar issues so I changed my power tubes to a match quad of JJ's and everything cleaned right up and haven't had problems since. Peace.
Music Man Distortion continued
I also use a mesa boogie V1 bottle rocket pedal between the guitar and the clean for a quick switch to smooth overdrive sound when I want it and it works perfect ....the pedal also has a pair of JJ 12ax7a's since my amp has the solid state preamp.
Sorted!
I've fixed the problem! All the voltages checked out OK, all the electrolytic capacitors were replaced and the ic's, speaker, and tubes were tested by substitution. The map was still distorted on the clean channel.
I found the two 470R resisters on the base of the 6L6s were reading incorrectly - one was 1k and the other was about 800R. I replaced these with so 500R resistors I had that were closer to spec than were in there, and that didn't fix it (I really thought I'd found the problem though!).
Last option was the transistors - I sourced three 2N6488s as I couldn't find any matched ones. When I tested them, their hfe values were 32, 34 and 44. I chose the 32 and 34 and put them in. Bingo! Beautiful clean, clear and powerful tones. The reverb issues I had were resolved as well - that sounded good too.
So problem sorted - it sounds better than it ever has, which is a massive relief!
As I was tidying up, I decided to check the old 1692 transistors - hfe values were 100 and 140 - seems a big difference to the 32 and 34 I replaced them with - but it does sound good.
Thanks to everyone and their suggestions. I thought I'd update the thread in case someone has a similar problem.
While I was there I added a Velcro strap for the footswitch as I'm not happy with that rattling around next to the Evm speaker. Thinking of putting a lower gain tube in the limiter circuit, as I've seen suggestions that it improves the tone, but I don't use that channel much so I'm not too bothered.
Time to play the hell out of it and put the Hot Rod Deluxe up for sale! I thought I'd upload a photo of my MM just in case anyone's interested. Not in the best condition but it has to be my favourite amp ever.
2N6488 success
I made the same change from the 1692 transistors to a pair of Motorola 6N6488s earlier this year in my 112RD-100 with good success. New Tungsol 6L6GC STR at the same time and it is running nicely. I was not as careful about finding a matched pair of the transistors...hadn't thought that would be necessary...but it seems like a wise plan.