midwayfair

Hissy reverb circuit in a Tone King Imperial
« on: February 07, 2019, 12:46:28 PM »
I've had this amp for a long time, but it's been a while since I plugged it in (I tend to use a 5W that I built to be a tiny version of it). I loaned it to my dad since he decided to buy some electrics recently, and I noticed that the reverb hiss is much louder than I remember. I always remembered it being noticeably hissy but I think it's quite loud now.

I've replaced the reverb driver and receiver tubes; the driver is specced as 12AT7 IIRC, but I've tried some 12AY7 and 12AX7 that I know are quiet in the other positions in the amp. The hiss sounds sounds the same except for the total output, so I don't think it's the tube. However, it does lead me to believe that the problem exists before the reverb out amplification stage.

Here's the schematic (this is the 20th anniversary but everything except the tremolo is identical I believe):

http://schems.com/bmampscom/tone_king/20th_Imperial_SC200HB_pg1.pdf

The power supply for the reverb is shared with the preamp, so if it was a PSU issue it would be in the whole amp. I'm not sure what else I can check or what would cause hissing.

Any ideas?
I'm Jon. Myself's music and things I make: jonpattonmusic.com. My band: www.midwayfair.org. [Disclaimer: PCBs of guitar pedals I've designed are sold by Madbean, 1776 Effects, and JMK PCBs.]


bibi

Re: Hissy reverb circuit in a Tone King Imperial
« Reply #1 on: February 07, 2019, 02:57:35 PM »
V4 is the reverb return tube.  That is the one to change first.  It appears to be a 12AX7 by the schematic, but actually a 12AT7 could work better, and if you really need less gain, thus less hiss, try your 12AY7 or a
12AU7.
If that doesn't help, as you mentioned it might not, I would imagine the builder probably used some super mojo carbon composition resistors for the plate resistors.  One of these (100k) in the reverb return circuit could be bad and causing excess noise.  A suitable 1/2 Watt metal film resistor would work just fine with less noise.

I doubt the driver is the source of the hiss as a spring is pretty frequency limited.

PRR

Re: Hissy reverb circuit in a Tone King Imperial
« Reply #2 on: February 07, 2019, 03:48:42 PM »
Has the tank been un-plugged, then re-plugged backward?

midwayfair

Re: Hissy reverb circuit in a Tone King Imperial
« Reply #3 on: February 07, 2019, 04:44:07 PM »
Thanks for the advice, I'll double check the tank.

I would imagine the builder probably used some super mojo carbon composition resistors for the plate resistors.

This ^ is is a massive mischaracterization of Mark Bartell's work. The thing's completely modern inside and uses metal film for all the resistors. It's been on a PCB since forever. He's really not into mojo.
I'm Jon. Myself's music and things I make: jonpattonmusic.com. My band: www.midwayfair.org. [Disclaimer: PCBs of guitar pedals I've designed are sold by Madbean, 1776 Effects, and JMK PCBs.]

bibi

Re: Hissy reverb circuit in a Tone King Imperial
« Reply #4 on: February 08, 2019, 02:34:18 AM »
"completely modern inside"? maybe, maybe not...
here is a photo of a Tone King Imperial with carbon composition resistors.
http://www.offsetguitars.com/forums/viewtopic.php?t=56827
here's an Imperial with a mix of resistors
https://www.thegearpage.net/board/index.php?threads/are-tone-kings-handwired.799666/

Maybe post a few photos of the amp's insides as well.  Any resistor can go bad,
so check those on the plates of the reverb return regardless of what type they are.

abbey road d enfer

Re: Hissy reverb circuit in a Tone King Imperial
« Reply #5 on: February 08, 2019, 04:15:26 AM »
Is the hiss level dependant on the reverb volume?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Mylithra

Re: Hissy reverb circuit in a Tone King Imperial
« Reply #6 on: February 08, 2019, 07:33:59 AM »
Most of the hiss Ive seen in tube amp has been caused by carbon comp resistors.  If its excessive, one might be damaged. You might try "chopsticking" around the reverb recovery and see if any of those parts make noise when you tap them. I'd consider those suspect.

Even if chopsticking comes back ok, Id still consider replacing the plate and cathode resistors of the reverb recovery.

*Chopsticking is dangerous since you have to do it with the amp on. Please take all necessary safety precautions and, please, don't use a metal chopstick ;P

midwayfair

Re: Hissy reverb circuit in a Tone King Imperial
« Reply #7 on: February 08, 2019, 09:15:12 AM »
Is the hiss level dependant on the reverb volume?

Yes, completely. (This is why the first thing I reached for earlier was the reverb driver. EDIT: Crap, did I read the schematic wrong? Is the driver the doubled stage before the transformer, not the two stages in succession after?)

I'll start poking around with chopsticks. At least if it is a resistor it'll be easy enough to replace.

I'll snap a photo when I'm inside, but I could have sworn the first time I had it open being surprised that it already had film resistors everywhere. I can't remember why I had it open, I might have been tracing the circuit for the mid bite to ste ... uh, get inspired by it.
« Last Edit: February 08, 2019, 09:20:02 AM by midwayfair »
I'm Jon. Myself's music and things I make: jonpattonmusic.com. My band: www.midwayfair.org. [Disclaimer: PCBs of guitar pedals I've designed are sold by Madbean, 1776 Effects, and JMK PCBs.]

abbey road d enfer

Re: Hissy reverb circuit in a Tone King Imperial
« Reply #8 on: February 08, 2019, 10:03:53 AM »
Yes, completely. (This is why the first thing I reached for earlier was the reverb driver. EDIT: Crap, did I read the schematic wrong? Is the driver the doubled stage before the transformer, not the two stages in succession after?).
That's correct, but I would rather suspect the recovery stage. Just to make sure, disconnect the cable that goes to the tank's output and short it. If noise is still there, the first half of the recovery stage is likely the culprit (U4b).
As PRR suggested, the tank may have been incorrectly connected; the schematic shows Reverb In, that should actually be connected to the tank's output, and Reverb Out, that should be connected to the tank's input.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

midwayfair

Re: Hissy reverb circuit in a Tone King Imperial
« Reply #9 on: February 08, 2019, 10:21:07 AM »
That's correct, but I would rather suspect the recovery stage. Just to make sure, disconnect the cable that goes to the tank's output and short it. If noise is still there, the first half of the recovery stage is likely the culprit (U4b).
As PRR suggested, the tank may have been incorrectly connected; the schematic shows Reverb In, that should actually be connected to the tank's output, and Reverb Out, that should be connected to the tank's input.

I swapped the wires. It's the same.

However, I just got it back to my house, and it's less hissy! Maybe it's the wiring in the 'rents house? I'm definitely going to poke around inside now when I get some more time.

Thanks the help folks. :)
I'm Jon. Myself's music and things I make: jonpattonmusic.com. My band: www.midwayfair.org. [Disclaimer: PCBs of guitar pedals I've designed are sold by Madbean, 1776 Effects, and JMK PCBs.]


 

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