HELP "65" Fender twin reverb

[conky]

First time inside an amp. Several months ago I replaced all tubes with new old stock american made tubes such sylvania, phillips JAN etc, seven 25uf/25volt caps, the bias cap back to factory specs with a 47/100v and resoldered three corroded pot connections.

THe 6l6's were sylv. matched quad marked 38. The bias was a snap to reset to the 38.

When first plugged the amp made some strange loud sounds, I found the reasaon was the bias cap replacement. For some reason the non-factory spec bias cap was the culprit. I put the bias cap I took out back back in the amp.

To my amazement the amp preformed way above my expectations, not the slightest hum, the clear channel bright and clear and the distortion outrageous.


Its my teen daughter amp and after about two weeks of hard use, she told me the amp was loosing volume and after about a week quit all together. I have the back cover off and when I power up the amp, I get a very dim glow only the farthest left power tube . all other tubes are dead.

I am limited in the test equipment department. Have multimeter and homemade bias checker.

Can anyone give me some information on what is faulty in the amp? Or where to start troubleshooting to pinpoint the problem?

Thanks
Conky

I see the asking prices for these original twin on E-bay are outragous. Possibly for economic reasons alot of folks are black facing the silver face amps.
A local pawn shop in the seedy part of town in Northern California has a original 66 Fender twin reverb for sale. Tolex not bad shape, grill cloth replaced hash job, but lots of extra material wrapped around the speaker board. Has replacement foreign tubes and appears or may have a replacement back chassis panel. Overall alot better shape than the amp I started with. I have the serial number, asking price 1695.00, may be negotiable. They told me its been in the window a long while.

[PRNDL]

THe 6l6's were sylv. matched quad marked 38. The bias was a snap to reset to the 38.

In general, you set the bias based on the anode voltage the tubes are getting, which in a Twin is about 400 volts, so 38 mA is fine.

Did you change the power supply electrolytics?
They are in the metal box underneath the amp. First discharge them carefully, then measure their resistance, which should rise as the DVM charges them.
You can also turn the amp on and measure the power supply voltages they are supplying, which should be close to the schematics for that amp.

Most likely one or more have gone bad. If so, it's best to replace them all.

As for the pawn shop Twin, you have to check the internals to see if it has original parts that are in good condition. These days, Twins aren't as desirable as other Fenders with less power (2 tubes instead of 4), which means that price is a bit high for todays market, unless the amp is in perfect condition.

[52 Bill]

What "65 Twin" is this amp, a real blackface, a reissue or a modded silverface?

When you say that only the far left power tube lights up, do you mean the orange glow of the tube filament? Does the pilot light, light up?

Unless this is a reissue, the filaments are string wired from left to right as you face the back of the amp. If there is a cold solder joint or a broken wire on the first power tube socket, all of the tubes to the right of it will not light up. If the tube fillaments are not heating up, the tubes will not work. So I'd check this out first.

[Enzo]

My read on the heaters is a short across the 6VAC. Look at the pilot lamp socket, are the leads touching? or is either side touching the frame? Or possibly the two leads shorted together on one of the sockets.

Pull a tube from its socket and measure between the heater pins - is there 6VAC there?

Without heaters running, all the caps, biasing, and tweaking in the world won't get sound out of it.

By the way, the number marked on the tube was the current - or something - that was read out on the matching machine. We have no idea what the electrical conditions were they used for matching. That number is NOT a recommended setting. it is just a coincidence it worked out in your amp.

[conky]

HELP "65" Fender twin reverb

Thanks PRNDL, 52 Bill and Enzo,

This is a second reply as my first reply has not posted.

I will use this information to preform the voltage checks. THe amp is a original 65 twin. The pilot glowed as well as one tube. I did not change the power supply electrolytics. The 38 was the number I came up with from a tube voltage graph I plugged my numbers into. I have pulled the chassis from the amp. Changed back to the old tubes and all tubes glowed and volume returned. Turned amp off and back on and three tubes glowed no volume. I pushed down on each tube and they all glowed again. But when switched to standby and back the same problem resulted a few tubes glowed. My first thought was the tube sockets are bad. They have been replaced before I purchased the amp. Looking into the sockets all metal pin retainers are bright and springy and appear to grip the tubes well. I noticed when the amp is switched back and forth from on to standby a bright blue flash come from the power tubes.

I will preform the teasts you guys recommended. And possibly with the additional information in this post will narrow the problem somewhat and some additional information will be posted to help in my troubleshooting my amp.

Sincerely
Conky

[Steve Conner]

Sounds like whoever replaced the tube sockets also screwed up the heater wiring. A bad solder joint, or a strand of wire touching the chassis or whatever. Check all the solder joints on the heater pins, or maybe just reflow them all as a matter of course.

If you find any bad ones, then it wouldn't hurt to resolder every pin on every socket, since you know the guy's soldering skills sucked.

PS, the "strange loud sounds" may have been because you fitted the replacement bias cap backwards?

[conky]

HELP "65" Fender twin reverb

Thanks Steve,

I replaced the bias cap when I retubed the amp. In addition, from information from the Hoffman library I also replaced the 7 - 25uf/25volt caps located in the circuit board.

I really do not what made the strange sound when I first fired up the amp. The only thing I could find was the bias caps per spec should have been 470ohm/3watt and 47/100V. I can't recall now will have to get at the bias caps to see what valued caps were in the amp, for certain their values were way different that Fender Specs. In addition I installed the bias caps from Hoffman and after the strange noise, when removing them to reinstall the original caps. I then noticed the Hoffman caps were also not the same value as Fender specs. I know the caps I installed were in correctly, I think by installing the hoffman caps which were of different value than the caps I took out caused the problem. I put the originals and the amp performed pertfectly, it never was so quiet since we owned it. You would have to look at the pilot light to see if it was running. When my daughter played through it I was floored by the tone, in the clean and distortion channel. I was ready to see the amp and look into a Matchless 30 watt, butr after the retubing of the twin there was no need to. I an new to working tube amps and possibly the bias cap specs fender show in a medium point and the caps need to changed out to able the amp to bias properly. Correct me if I am wrong.

I thought I had a tube socket problem but now with two replies regarding the heater wires I should concentrate on the heater system. I had the amp in a shop to be checked out when I purchase it. Close to $400.00 of work was done to the amp, mainly cap replacement to bring the amp back to fender specs as it had been moded at 25 watts. I recall when I was at the store to pick up the amp. My daughter looked at each other as from the back room a loud electrical surge, current whastever that sounded like the Frankenstein mosnsters shock back to life. Then the owner of the store came out from back and said that was our amp as they found another short to fix.

The played well after the repairs, but always had a hum, if my daughter let on unattended I was able to hear it and go in her room to shut it down.
I found that no main ground lug was on the amp. Desoldered the wires from the chassis and installed a lug and soldered the lug to the chassis, it took two weller guns to generate enough heat to make the lug solder. Also going over the wires two wires from the potts just fell off in my hands when touched and another was not soldered well see the eye hole on the pot lug.

I think that wayt the amp performed well and was super quiet. The heater wires are of the original cloth braded variety, but look to good to be over 30 years old, and must have been replaced when the sockets were replaced.
They just look to good and solder very well in my opinion to be the problem.
But I'am new to tube amps. What I found is the tubes have to be pushed on and they will light and amp will play with hum, but when switched to standby or turned on and off, the tubed have to be pushed on again to light them. Is this a symtom of faulty heater wires or heater system? I will reflow them, they were soldered by who ever installed the sockets and have been soldering and welding since 8th grade and I am 60 years old. And I do not think I can solder as well as the guy wqho solder them.
Steve let me know what else could be the problem. The sockets are replacement of original black plastic variety original to the amp and look very good. Could sockets be a possibility for the problem or is my problem not something the sockets would cause. Maybe this a dumb question as they are going in and out as multiples not any one random socket time after time.

Thanks
Conky

[conky]

HELP "65" Fender twin reverb

Again thanks for your help and a special thanks for Mr Hoffman for the wealth of tube amp information posted on his website. After some thought and a review of Servicing Fender Amps section from the Hoffman library. I was able to grasp why my amp is having this problem.

I since with a magnafine glass followed the heater circuit checking all the connection and making sure the heater wires are up and straight down to the terminal connection. Then I fired up the amp and it have performing with no problems for the past hour.

My daughter plays thru the Fender amp. She had the amp since was 13 and played the amp atleast 50 hours or more per week. Myspace/the backbeat band, a song is posted she wrote and performed as singer/lead guitarist at 13 years of age.

Thanks again
Conky

[MWJB]

"The 38 was the number I came up with from a tube VOLTAGE graph I plugged my numbers into." If you are saying that you have been running the tubes at -38vdc at pin 5, then you have most likely burned out the power tubes. Set your bias by measuring mA at the tube plate at idle. A BF Twin will run well over 400v (probably nearer 500v) if in good order and fitted with the original PT, I'd keep plate current to the early 30-ish.

As Enzo says, unless that "38" written on the tubes was written after setting the bias in your amp specifically, there is no reason to expect that it has ANY relevance whatsoever to running conditions in your amp.

Anything up from 25uf-100uf/100v will work as the bias cap & will not affect noise at the higher end of the uf scale. As Steve says, either the new cap was installed backwards, poorly grounded or a faulty part.

We also need to clarify what is "glowing" in your tubes:
Yellow points of light at top & bottom is heaters & good.
Blue gassy glow that increases when standby is thrown & when you play through the amp is normal.
Any red or orange glow in the power tube's large grey/black plates is bad, very bad.

The fact that your power tubes need to be pushed into the sockets to work means that over the years, with plenty of tube swapping, the sockets have lost tension and are not gripping the tube's pins properly. This is very common. Power down the amp, drain the filter caps, remove the power tubes and bend in the contacts inside the tube sockets until they grip the tube pins firmly, clean with a little isopropyl alcohol on the tube pins and work them in/out of the sockets a few times. When done check your work by powering up the amp, all volumes/tone controls to full and gently rock the tubes in their sockets - any weird noises, repeat the process, replace any sockets that can't be retensioned.

Spring retainers that cradle the bottom of the tube are not going to help/cure this problem, they will ony ensure the bases are pushed up against the socket, not that the pins are making good electrical contact with the socket's contacts. Spring retainers are not usually necessary (unless you do a lot of off-roading with your amp in the trunk), the fact that they have been fitted to an amp usually means that someone has tried a stop gap measure to cure loose sockets. Tight sockets and the raditional Fender "bear trap" retainers should work fine. The store should have done this as part of that $400.00 (oweee, oweee, ouch! I would only expect a bill that big if you had a new transformer or speakers fitted?) worth of work, it's one of the first things I do with any old amp.

[Rob Mercure]

It's kinda hard to follow all of the work that has, or hasn't been, done to the amp but MWJB, Steve, and Enzo's advice is good but if I understand you had $400 worth of work done on the amp and the power supply filter caps haven't been changed? If so, if you'll pardon the expression, it seem like someone "burned you an new ___!" For that price you should almost be able to replace the entire circuitry.

And I wonder about the tube sockets being replaced. While I've occasionally replaced one tube socket on a Fender amp I've never had to replace more than on a particular amp - and I assume that these are the octal sockets for the rectifier and output tubes? While other's may have had to I've never replaced a miniature socket on a Fender.

One thing I think that confused MWJB's response is your use of the term "retainers" when I believe you mean "contacts" - those that connect the tube pins to the corresponding leads.

Are you familiar with using a DMM (being 60 years old perhaps at one time a VTVM - I'm 54)? If so you should be able to detect 6.3 volts AC (more or less, between 6 and 8 say) on pins 2 and 7 of the output tubes sockets - on every socket - this is the heater voltage that "lights up" each output tube with a rosy orange glow. To determine the pin numbers from the side you plug the tubes in you locate the tube base "key" - the little slot in the center hole - and count counterclockwise - pins 2 & 7 are one pin hole from the key on each side. For the miniature tubes you should have the same voltage reading - or close if you leave any of the output tubes - as you have on the output tubes. You again read the miniature sockets counter clockwise from the tube side and the heater voltage should be from pin 9 to either 4 or 5 (their tied together). Also, you should check the output tube control grid bias: set the DMM to DC volts, touch the black (-) lead to ground and when you touch the red lead to pin 5 you should read around - 50 volts DC. Make sure that the voltage there is negative - if not you'll fry the output tubes and, as previously recommended, make sure it is around 50 volts. Also, as recommended, you should set the bias based on the current flowing through the output tubes but if you're not prepared to do that then set the bias voltage as high as it will go - again making sure that it appears at pin 5 of each output tube and that it is identical for each tube.

For the extensive work that must have been done to your amp I assume that you received a written description of the specific work and were offered the old parts removed from the amp? If not I recommend that you return to the store that did the work and demand such! If you've already got this, or after you get it, if you don't mind please post an concise description for us - I'm truly interested in what could have been worth that much money as I've never had to do over $300 in work on a Fender - this also being a Twin that had to have both the output and power transformer replaced (and I suspect that they did so if someone had converted it to 25 watts output - or at least the power tranny).

Oh, and let us know what the voltage readings I've suggested are.

Rob

[Rick Erickson]

[the fact that they have been fitted to an amp usually means that someone has tried a stop gap measure to cure loose sockets.]

Or that the amp was once fitted with the original short-base Sovtec 5881 power tubes. Using those tubes with the base gripping Frnder spring retainers was/is a sure fire way to crack the tubes where the metal "grips" touch the glass. The new Marshall style chrome plated retainers with the chrome plated springs are just as bad. I've seen many amps with cracked tubes from the extreme tension of the plated springs. Aparently the Yellow Cadmium (or Zinc) Plated Marshall style retainers that New Sensor used to sell are no longer available. The springs on those were soft and springy, not stiff like the new ones. The new ones look real nice and shiny though... sigh

RE