It is currently Mon Mar 16, 2020 5:50 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 31 posts ]  Go to page Previous  1, 2, 3  Next
Go to page Previous  1, 2, 3  Next
Author Message
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Thu Jul 04, 2013 4:09 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Sun Jun 16, 2013 2:37 pm
Posts: 24
Location: Seattle, WA
Finished rebuilding the amp: new tube sockets, new bias pot, new circuit boards & components, I left all the control pots. The problem still persists!! If I have the volume past 3.5ish and the bass past 2 I get nasty farting distortion, and the volume seems to be low because I can turn it up to about 7 without plugging my ears. I reiterate, the problem is the exact same as before the partial rebuild. I am glad that I rebuilt this amp because it was fun and I learned a lot and I will probably build another from scratch eventually, but damn I have put a lot of time and money into it. Is there a way to test the output transformer? A tech told me that it could go bad if there was ever and arcing event, and my power tubes were arcing a while ago when they were way out of balance with each other. All tubes and resistors have been replaced since then and the bias properly adjusted.

Here are some pics, I have been experimenting with the normal channel for a good bass guitar sound, so that is not standard. The problems I have described apply to both channels but I am mainly analyzing the vibrato/reverb channel since that is all I use for guitar. Thanks for the help guys.
Image
Chassis by esbkeysfan, on Flickr
Image
Power Amp Section by esbkeysfan, on Flickr


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Thu Jul 04, 2013 7:13 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Nice job! Farting is usually a gain stage coupling or idle bias issue. Allowing too much bass to pass and be amplified in a stage. And/or problems in local or global feedback.

Prolly need to go through each stage and try to isolate the issue.


Good luck. Keep us posted.


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Thu Jul 04, 2013 11:56 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
JZL wrote:
The problems I have described apply to both channels


That is telling you something.

Are you listening?

:roll:

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: 1975 Twin Reverb Rebuild
Posted: Fri Jul 05, 2013 3:19 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
You know, looking at your photo... gotta ask you: "What are the values of the three caps circled in red?"

They should be 0.02 mfd/0.01 mfd/ 0.01 mfd. Yours look very big for these values. Unless they are 1600 VDC rated caps. See the layout of AA270 (which, I believe is close to your 100W TR, in this section) to compare.




Image


Image


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Fri Jul 05, 2013 3:19 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
edit: duplicate entry. sorry


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Fri Jul 05, 2013 12:20 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Sun Jun 16, 2013 2:37 pm
Posts: 24
Location: Seattle, WA
Retroverbial wrote:
JZL wrote:
The problems I have described apply to both channels


That is telling you something.

Are you listening?

:roll:

Arjay

Not quite sure what you mean by this.

BMW2002Ti wrote:
You know, looking at your photo... gotta ask you: "What are the values of the three caps circled in red?"

They should be 0.02 mfd/0.01 mfd/ 0.01 mfd. Yours look very big for these values. Unless they are 1600 VDC rated caps. See the layout of AA270 (which, I believe is close to your 100W TR, in this section) to compare.

They are the correct values and they are 600 V.

I am just wondering what this could be assuming (I know it's not neccessarily true) that it's not on the circuit boards or the tubes. It seems that it leaves only the transformers as possible faults. Especially since it happens in both channels and the volume isn't completely unbearable when it's cranked up.


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Fri Jul 05, 2013 11:36 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
JZL, have you measured the B+ tap voltages? And the anode voltage & idle bias on the 6L6GCs & the plate and cathode voltages on the 12AX7 and 12AT7?

Best to do this first before any tranny work. See the voltages on the schemo...

Bottom scan of the link's PDF file is a bit clearer than the top one:

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


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sat Jul 06, 2013 8:51 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
JZL,

Just to be real sure about those three Orange Drop coupling caps (0.02/0.01/0.01) ---->


My 0.2mfd/600VDC cap reads: 224K600.

And the SBE 0.02mfd/600VDC reads: 223J600

Is this what you have on those caps. That "0.02" cap sure looks like a 0.2mfd cap...


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sat Jul 06, 2013 9:19 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Sun Jun 16, 2013 2:37 pm
Posts: 24
Location: Seattle, WA
BMW2002Ti wrote:
JZL,

Just to be real sure about those three Orange Drop coupling caps (0.02/0.01/0.01) ---->


My 0.2mfd/600VDC cap reads: 224K600.

And the SBE 0.02mfd/600VDC reads: 223J600

Is this what you have on those caps. That "0.02" cap sure looks like a 0.2mfd cap...

I have another closeup of the picture and indeed it says 223J and says 600V. Soon I am going to take the chassis out again to double check everything and get some voltage readings.


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sat Jul 06, 2013 10:14 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
JZL, ok. Be interested in those readings.

Good luck!


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sun Jul 07, 2013 12:36 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Sun Jun 16, 2013 2:37 pm
Posts: 24
Location: Seattle, WA
BMW2002Ti wrote:
JZL, have you measured the B+ tap voltages? And the anode voltage & idle bias on the 6L6GCs & the plate and cathode voltages on the 12AX7 and 12AT7?

Best to do this first before any tranny work. See the voltages on the schemo...

Bottom scan of the link's PDF file is a bit clearer than the top one:

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

I took the chassis out and turned the amp on. As soon as I turned it off standby there was a bright and loud spark somewhere in the tremolo circuit. The tremolo is definitely bad now, it pulsates without any input signal and kind of clicks softly. So there is a new problem to add to my list. When the tremolo is off the amp sounds as it did before the spark.

So I took readings and here is what I got:

B+ taps - 332 VAC (the red wires right? sorry I am new to this stuff)
Plate voltage on 6l6's - about 451V (using a Bias Rite)
Idle current on 6l6's - about 31 mA (using a Bias Rite)
V6 pin 1 - 245V
V6 pin 3 - 110V
V6 pin 6 - 252V

These all seem within reasonable limits to me. Now the bad news, here are the readings in the tremolo circuit:

V5 pin 1 - 437 V
V5 pin 3 - 0
V5 pin 6 - 393 V
V5 pin 8 - 10.2 mV

I replaced the tremolo tube and there was no difference, so I don't think the tube went bad. This is all starting to frustrate me (I've had so much patience for months now), maybe it's just been a long day. Any more help from you guys is greatly appreciated. Thanks a lot.


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sun Jul 07, 2013 5:19 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
JZL,

I know how you feel. Maybe it's better to take a break & come back with a fresh mind.




V5 pin 1 - 437 V
V5 pin 3 - 0
V5 pin 6 - 393 V
V5 pin 8 - 10.2 mV



After the break... look at what is attached to Pin1. Those 0.02/0.01/0.01mfd caps. These caps help determine the shift change in the vibrato circuit. The voltage should be closer to 250-300VDC.

And pin 8's reading of 10.2mV prolly means that there is no idle current through this half of V5. Voltage should be 15-20VDC. Pin 8 is the cathode of this half of that 12AX7.

You need to check the circuitry around your V5. There is something wrong. BTW... I've used the Sprague Atom 8mfd/250VDC for that 5mfd/25VDC bypass cap. Size is ok. Very stable cap.

There TR AA270 layout for the virato section (V5) is similar to your amp. Look at the bottom half of the link. Maybe the its layout and voltage values may help you find the issue with your amp.


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


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sat Jul 27, 2013 10:09 pm
Offline
Hobbyist
Hobbyist
User avatar

Joined: Sun Jun 16, 2013 2:37 pm
Posts: 24
Location: Seattle, WA
Okay so I took a break for a few weeks and played a few shows, now I am ready to get back to this Twin. Can anybody tell me how I can check the output transformer and choke, or lead me to information about doing this? So far there has been mention of doing this, but nobody seems to know how or maybe they just don't wish to tell me. For now I am going to ignore the recent tremelo problem because I don't believe this has anything to do with the original problem (and I don't even use the built in trem). Any advice before I go and buy a new OT and/or choke?


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sun Jul 28, 2013 8:30 am
Offline
Aspiring Musician
Aspiring Musician
User avatar

Joined: Fri Aug 21, 2009 1:45 pm
Posts: 852
Location: SoCal323
Just a thought but have you tried it through a different set of speakers/cab? Also, if the tech you took it to originally mentioned that the transformer might be bad, then that definitely needs to be addressed, specially if the guy has a good reputation.
Did a quick google search on how to test a power transformer for tube amp and it revealed several articles so I would suggest maybe the same. Here's a quick link to one of several.
http://www.ehow.com/how_7775846_check-o ... ifier.html
Hope this helps and good luck.

_________________
Its a lifestyle.....

Image


Top
Profile
Post subject: Re: 1975 Twin Reverb Rebuild
Posted: Sun Jul 28, 2013 9:35 am
Offline
Hobbyist
Hobbyist
User avatar

Joined: Sun Jun 16, 2013 2:37 pm
Posts: 24
Location: Seattle, WA
socal323 wrote:
Just a thought but have you tried it through a different set of speakers/cab? Also, if the tech you took it to originally mentioned that the transformer might be bad, then that definitely needs to be addressed, specially if the guy has a good reputation.
Did a quick google search on how to test a power transformer for tube amp and it revealed several articles so I would suggest maybe the same. Here's a quick link to one of several.
http://www.ehow.com/how_7775846_check-o ... ifier.html
Hope this helps and good luck.

Thanks for the link, I'll try this out later. I did try the amp through a couple different speaker cabs and still had the same problem.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 31 posts ]  Go to page Previous  1, 2, 3  Next
Go to page Previous  1, 2, 3  Next

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: