It is currently Mon Mar 16, 2020 8:27 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 11 posts ] 
Author Message
Post subject: Silver Face Twin Reverb - Reverb Issue - slight distortion
Posted: Mon Jun 18, 2012 2:01 am
Offline
Hobbyist
Hobbyist

Joined: Thu Jun 07, 2012 2:29 am
Posts: 22
Hello - I have a 1976 Fender twin Reverb with JBL's that I have owned and used from new. I recently did a re-cap on the amp and replaced all the Valves (Tubes), the original set still work fine, but decided they needed replacing. I have an issue with the Reverb Circuit that existed before any of the above work. The reverb very slightly distorts, it's worse the lower the Master Volume is i.e if on 10 and pre-amp volume set very low it's almost not noticable, but if the master is on 4 and pre-amp on 2 or 3, you can hear some distortion, almost after the note, so it's not out and out distortion. I have swaped the Reverb Valves (tubes) and checked the tank, all seem to be perfect. The Reverb Electrolytic is new. I have checked the resistance values round the circuit and they seem to be OK. I'm not sure what else to check - any suggestions? The amp is 1976, with Master Volume and the 135 watt Ultra Linear circuit, it is stock except for new Tubes and Grid resistors.

Thanks for any advice offered.


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Mon Jun 18, 2012 2:20 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Check the plate voltages and the ground-to-cathode voltage of the 12AT7 driver and the 12AX7 recovery tube of the reverb circuit. Sounds like maybe too high plate voltage or too high idle bias on those tubes.

http://www.webphix.com/schematic%20heav ... _schem.pdf


Top
Profile
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Mon Jun 18, 2012 6:03 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
BMW2002Ti wrote:
Check the plate voltages and the ground-to-cathode voltage of the 12AT7 driver and the 12AX7 recovery tube of the reverb circuit. Sounds like maybe too high plate voltage or too high idle bias on those tubes.

http://www.webphix.com/schematic%20heav ... _schem.pdf


+1

My money's on the V4 recovery stage.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Tue Jun 19, 2012 2:26 am
Offline
Hobbyist
Hobbyist

Joined: Thu Jun 07, 2012 2:29 am
Posts: 22
Thanks very much for taking the time to reply, the recovery tube could be the prime suspect I would guess, maybe the 100K 1 Watt resistor has drifted for plate voltage? Possibly the 820 ohm on the Cathode, all the caps are new, but it was doing this before they were replaced so I guessing a resistor has drifted.

Do you think any of the PSU resistors might have drifted as the caps are now new and working to full voltage? They looked OK when I changed the caps, but they are old, might not notice if the volts are higher in other parts of the circuit.

Might be a week or two before I can get in there to have a look, thanks very much for your help and suggestions.

Cheers
Simon


Top
Profile
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Tue Jun 19, 2012 7:05 am
Offline
Professional Musician
Professional Musician
User avatar

Joined: Tue Jul 17, 2007 3:08 pm
Posts: 2889
I went through something similar with my bassman head, i kept changing out of spec resistors until I ran out of out of spec resistors, all the remaining were within spec and yet I had a sizzle in the sound that was driving me crazy.
The fellas (sorry can't remember which of you) suggested that the old carbon comps can absorb moisture and give off a slight arching sound.
Sure enough a 100K carbon comp that tested with 10% was the culprit, all nasty sizzle went away with the swapping of that 45 year old resistor.
I did replace most of the carbon comps with new carbon comps so it wouldn't compromise the tone.

Goes to show that having a resistor in spec does not mean its not causing trouble !

_________________
Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


Top
Profile
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Tue Jun 19, 2012 7:59 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
blues bondsman wrote:
Goes to show that having a resistor in spec does not mean its not causing trouble !


+1

Sometimes it's the smallest of things that cause the largest of headaches.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Thu Jul 05, 2012 1:02 am
Offline
Hobbyist
Hobbyist

Joined: Thu Jun 07, 2012 2:29 am
Posts: 22
I finally had enough time to look at this problem, but still can not figure it out. All the voltages are correct from the Silverface 135 Watt Drawing. Took on board comments about resistors so replaced all resistors in the Reverb Send and Recover circuit, but still have this problem. I suspect it has always been like this and it's more a design issue?

If I put the channel volume on 2, master volume on any setting, reverb on any setting, the amp is perfect, with or without reverb. Put Channel volume on 5, master on any setting, the distortion is heard, mostly at the end of a note. Whilst the pre-amp is slightly breaking up, as this is the Master Volume version of this amp, the reverb distortion is different. It's almost like the reverb circuit is being overloaded, not sure why it is more noticeable when the note drops off, one other thing I noticed, if I pull the master Volume switch (Never used this or liked it), the problem is less noticable, you would think it would be worse.

I just got some new tubes, so changed these out and tested them in a different amp, all perfect and no change to the problem.

Not sure where else to look now, any suggestions, or do you think it's a circuit design problem and always been like this? I have to admit when I used the amp for gigging many years ago I never took the pre-amp above 2, so would not have noticed it, but styles change and now I use a more crunch sound from time to time so have just noticed the issue, 35 years latter ha ha

Thanks for any feed back provided

Cheers


Top
Profile
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Thu Jul 05, 2012 6:51 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
6B4G wrote:
I suspect it has always been like this and it's more a design issue?

Not sure where else to look now, any suggestions, or do you think it's a circuit design problem and always been like this?


I've owned a half dozen Twin Reverbs since 1970 (all versions, from AB763 blackface to '65 re-issue and everything in between) and none of them exhibit the symptoms you describe.

There is no "design" issue.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Thu Jul 05, 2012 7:18 am
Offline
Hobbyist
Hobbyist

Joined: Thu Jun 07, 2012 2:29 am
Posts: 22
Thanks Arjay - so it confirms it still has a fault, any suggestions as where to look noting that all voltages static read correct, and I have changed all the obvious resistors etc. I inspected the springs etc. and they are fine, plus with lower pre-amp volume settings it sounds perfect up to very, very loud volumes. It does seem related to the setting of the pre-amp volume, however with the reverb turned off the amp is perfect, no hint of distortion or the problem. Strange problem this one. I don't have a scope to go chasing this, only a DVM so somewhat limited in what I can check for.

Thanks for any suggestions
Simon


Top
Profile
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Thu Jul 05, 2012 7:38 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
You do have the correct tube types installed in your V3 and V4 sockets, right?

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Silver Face Twin Reverb - Reverb Issue - slight distorti
Posted: Thu Jul 05, 2012 7:47 am
Offline
Hobbyist
Hobbyist

Joined: Thu Jun 07, 2012 2:29 am
Posts: 22
Hi Arjay,
Yes - went through all tubes to make sure they are the right type and in correct position, and changed them between different amps to make sure I don't have a faulty tube lurking in the circuit somewhere. All sound fine in another amp in different positions, I'm almost 100% sure it's not a tube, never say 100% but 98.99% sure it's not a tube.

Thanks


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 11 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: