It is currently Mon Mar 16, 2020 3:34 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 21 posts ]  Go to page 1, 2  Next
Go to page 1, 2  Next
Author Message
Post subject: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sat Nov 22, 2014 4:34 pm
Offline
Amateur
Amateur

Joined: Sat May 25, 2013 10:02 pm
Posts: 197
Hi All - Gotta figure out why i have low output. Not distorted, crackly (bad screen grid resistor fuzz). Its clean and loud but not as should be. checked and tested all tubes on equipment.

I just dont feel that its pushing ALL THE POWER these beasts are known for.

All caps read in spec, no funny noises, no crackles....NOTHING.

Any suggestions where to look?


Thanks


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sat Nov 22, 2014 6:05 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
1.) Check your 6L6GC idle bias. And plate voltages. Post results. Should be around the 16-20 watt per tube range... where:

(Wattage) = (plate volts, VDC) times (idle bias current, in amps)

example: (18 .8 watts, per tube) = (470VDC) times (0.040amps, or 40mA)


2.) How old are those main filter caps? They may test ok, at idle, but lack energy reserve under load --- if they are too old.


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sat Nov 22, 2014 9:20 pm
Offline
Aspiring Musician
Aspiring Musician

Joined: Sun Oct 27, 2013 7:11 am
Posts: 318
This may sound "out there" but check the voltages on V6.


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sun Nov 23, 2014 3:26 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Yeah! Just try a new 12AT7 (or known good one) in the phase inverter position. One half of the duo-triode maybe weak or dead. The amp will still play, but have a noticeable change in bass quality and transients.


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sun Nov 23, 2014 5:27 pm
Offline
Amateur
Amateur

Joined: Sat May 25, 2013 10:02 pm
Posts: 197
There is 475v on all 4 of the 6l6's

But on v6 (PI) it gets weird

Pin 1 = 066

Pin 2 = 284

Pin 3 = zero

Pin 4 = 105

Pin 5 = 062

Pin 6 = 293

Pin 7 = zero

Thanks for chming in BMW


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sun Nov 23, 2014 5:50 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Pin 1 should be in the 200VDC range. Pin 2 should be close to zero. 4 & 5 and 9 are the heater pins. DC readings on these will not mean much.

Maybe, re-read the values, again? Or just try another 12AT7. If you don't have a spare, pull the reverb driver (V3) and try it in the V6 position. See if that changes your tonal response. The amp will work in NORMAL channel, with V3 pulled.

Good luck! Post results. :)


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sun Nov 23, 2014 8:12 pm
Offline
Amateur
Amateur

Joined: Sat May 25, 2013 10:02 pm
Posts: 197
http://i7.photobucket.com/albums/y286/6 ... 1181f5.jpg

I appears to me that there are only 7 pins because 1 & 6 are soldered together and so are pins 4 & 5????

Re-reading as follows

Pin 1 (tied into pin 6) Zero

Pin 2 = Zero

Pin 3 = 432

Pin 4 Tied into Pin 5 = Zero

Pin 6 = (as noted above tied into Pin1) Zero

Pin 7 = Zero

Pin 8 = 432

Pin 9 = Zero

Thanks Guys


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sun Nov 23, 2014 11:08 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Hmm... it almost sounds like you are measuring the voltages on the reverb driver tube (V2) and not the phase inverter (V6).

V2 has 1 & 6; 2 & 7; 3 & 8 tied together.

V6 has 3 & 8 tied together.

Both have 4 & 5 tied to each other.


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sun Nov 23, 2014 11:27 pm
Offline
Amateur
Amateur

Joined: Sat May 25, 2013 10:02 pm
Posts: 197
No Sir PI is right next to the first 6L6 (v7)

I wish I was wrong.


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Sun Nov 23, 2014 11:31 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Pins 1 & 6 shouldn't be tied together. The PI would not work properly. Did someone modify the amp?


http://bmamps.com/Schematics/fender/twi ... _schem.pdf


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Mon Nov 24, 2014 8:07 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Ryan41018,

Sorry, I posted the wrong schematic for your amp. Your is the UL 135-watt Twin Reverb model. The PI and reverb driver sections (V6 and V3) seem similar to the 100-watt model schematic that I posted earlier.

http://bmamps.com/Schematics/fender/twi ... _schem.pdf

Try pulling the 12AT7 in V3 and put it into V6 and see if that helps. Or try a new 12AT7, if you have one.


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Mon Nov 24, 2014 8:25 am
Offline
Amateur
Amateur

Joined: Sat May 25, 2013 10:02 pm
Posts: 197
I have swapped out v6 PI to "new" ones. The amp is a 1973 and is a 100 watt version not 135 watt.

Frustrated....


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Mon Nov 24, 2014 10:01 am
Offline
Aspiring Musician
Aspiring Musician

Joined: Thu Dec 12, 2013 2:54 pm
Posts: 619
From the picture you posted, it appears that pins 3 and 8 are tied together, not 1 and 6.

It's hard to tell from the picture, but make sure those contacts on the speaker jack open up completely when the plug is inserted. If they don't, that would cause a reduction in volume. (a fairly drastic one at that)

You haven't answered Beemer's question about the age of the electrolytic caps. If they are original to the amp, they need to be replaced.

vinyl

_________________
"Where confusion abounds, there's money to be made."


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Mon Nov 24, 2014 10:08 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Ryan41018 wrote:
I have swapped out v6 PI to "new" ones. The amp is a 1973 and is a 100 watt version not 135 watt.

Frustrated....



I guess no change with the "new" 12AT7. Check those 'lytics. Anything over 25 years old is suspect. Even if the amp was rarely used.


Top
Profile
Post subject: Re: 1978 Twin Reverb - Master Volume-------HELP
Posted: Mon Nov 24, 2014 11:05 am
Offline
Aspiring Musician
Aspiring Musician

Joined: Sun Oct 27, 2013 7:11 am
Posts: 318
+1 on the cap job. Even if they're not the culprit, if they're old they need to be replaced regardless.

UN


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 21 posts ]  Go to page 1, 2  Next
Go to page 1, 2  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: