1966;67;68 Twin Reverb SF Drip Edge Black Line DungHeap

[Jolsen]

Came across this very special amp and payed more for it than I wanted to. Took it home and immediately plugged it in, turned it on and immediately turned it off. It made more noise than any amp I have ever owned. I decided to make it my learning vehical. I have played guitar for a very long time and presently own more amps/guitars than my wife thinks is healthy. She only picks on me once in a while. I havent done any serious work on any of these amps. but I changed the big caps in the doghouse on this TR. I changed the 25/25 caps out too. I believe these are coupling caps? correct me If I am wrong. I bought a variac and slowly brought this amp back up to 120vac. Still hisses and pops ungodly. It has power tranny from middle 1966, 606-6-35; The choke is 606-6-30; output tranny is 606-7-03 and the reverb choke/tranny is 606-7-15. Pots are from late 66 early 67 as well. A spare parts machine for sure. I cleaned the pots with spray and twist technique...puretronics contact cleaner. Still hisses and pops but I notice it is only the vibrato side that has a real problem.
I stopped here and cleaned the crap out of this amp. sow bug shells; nicotine; layers of filth.
It made me feel "not so ineffective".
Got my old meter out and started poking around. This amp is 45+ years old. The resistors I know to be concerned about.. 1500 on the 6l6cathodes and the 470 off the grids of same. All of these resistor read 47years high. 1880/1910 or so and 560 up towards 600. I notice that 6l6 470 ohm resister on output tube #2 looks a little smokey. found corresponding black sooty spot on faraday screen in cabinet. blown tube at one point for sure....Houston? We have another problem besides the pop and hiss? This amp is just old. Then again all my amps are old. vibrato side tone caps are not known for failing right? should I just change these out. Are there any other more rational avenues to pursue here? I'm definately changing the 470 and 1500 out.

[Reeltarded]

The board sucks moisture. The plate resistors need changing. Clean all sockets and jacks. Reflow bitchy grounds.

Pin your meter to ground and probe around on the board for stray DC voltage. If you find it your board is doing 'that thing where they'.. and lots do.

[Jolsen]

lookin at the tube pinout, plate resistors are 100k guys associated with pins 1 and 6. of the preamp tubes. 12ax7 for the vibrato section.. I guess this is preamp section 2. I will check em out.
When you say probe the board you mean the actual board itself, the phenol or whatever it is made of; eyelet board. I thought these were good insulater type material. Man I would hate to change out an entire board.

[Reeltarded]

Yep, and yes. Just check it. Before you do too much of anything swap all the tubes for the Vib side out with any that you know are good.

Do your pots make scratchy noises at all? Do you feel even the smallest tingle if only the skin of your wrist grounds to the bridge or a string? If you are probing around and drag the probe across the board there might be an audible frittering with pops that is obviously different and caused by you doing that. That would also point to DC on the loose even if you can't get a reliable reading of how much or exactly where/why.

You need a can of freeze spray. Very small amount right on board components while the amp is warm and running. Just a tiny tap with the extension tube in for control to search for the culprit. Do that with that volume moderated.

Umm.. previous tube failure. With the amp off and power tubes removed check the values of those resistors on the output sockets. i would expect both of the screens to be about the color of medium brown eggs.

If everything seems good it's time to start chop sticking at all the solder joints. Could be as simple as a reflow on a joint or two.

Someone with better ideas will be along shortly to berate me for idiocy for not thinking of the most obvious.

[martin manning]

The board material Fender used is called fishpaper, and it has been known to absorb moisture over time and become conductive. Probe the board around the eyelets with high DC voltage on them and see what you can find out.

General cleaning should include all the pots and jacks as Reeltard said, plus the tube sockets. Get yourself some Deoxit D5 for sockets and jacks and some F5 fader lube for pots. Put the D5 on the pins of each tube and insert and remove it a few times. Spray F5 (sparingly) into the pots and rotate them stop-to-stop ~10 times. Radio Shack also sells a control and contact cleaner that seems to work well.

The 25uF 25V electrolytic caps are bypassing cathode resistors, and so they are commonly called cathode caps. The screen and control grid resistors on the power tube sockets are probably 10% tolerance parts, so that can be used as a criteron when deciding whether to replace them or not.

If after cleaning the jacks, pots, and sockets you still have hiss and popping noises suspect the plate resistors on the preamp tubes. These can all be metered in-circuit to check for drift.

[Jolsen]

Thanks for the help. This is a bizarre amp. It was sold as a '68. Ahh the formative years. I have a 1967 BFSR with trannys older than this 68twin.

[Jolsen]

What I thought I said.... The 67BFSR has trannies that are.... oh what the heck. The reputed 1968 dripedge Twin was assembled before the 67 BFSR.

[Jolsen]

I started checking resister values. plate resistors in preamp sections are all just at the 10% limit or worse. All resistors in the phase inverter are toast. I checked all of the 5% resistors here and they are just BAD. at the 12at7 the plate 82Kohm guy pin1 reads 185Kohm; and the plate pin 7 100kohm resistor reads 125Kohm. The 470ohm cathode resistor pins 3and8 reads 755ohms. output tubes ..the 470 1Watters or 50% out of spec and the 1500 ohm grid resistors or all over 2K. This thing got real hot at some point.
Waiting on carbon comps now. Man these things got expensive. I have lots of projects now that I'm retired. I'm glad I haven't put any real good tubes (read expensive) in this amp yet. sticking with the grovvetube 5881 sovteks it came with. I must get it correct first. This silverface is the AB763 circuit. 66 and 67 trannies, a 1968 mutant for sure.

[Jolsen]

Thanks for the advice in this effort. I checked the guitar and ampface for sizzle and felt no stray dc at the strings, pickups, or ampface. I didn't want to leave it on long enough to probe the eyelet board. Im goin to try the new cleaners in the pots and other prescribed areas while the amp is down. I also have a sick 67 Dualshowman head. I need a replacement OT. I have output comparable to a 50w bassmens db level. suggestions? I am not to confident with this part of the amp yet and I would like to try an exact replacement. Help! tubes are known good. resistors at the output sockets all changed out.

[martin manning]

You do know that you can't measure many of the components in-circuit, right? Some, like plate loads, grid stops, and screen resistors you can, because one end is unconnected if the power is off. Others you will have to lift one end from the circuit board. Plate load resistors can be in-spec but noisy. Carbon comps are the worst offenders there.

[Jolsen]

Thank you for the advice. I found that out last nite while removing resistors. I went so far as to dig out an old meter as a second check. Older and Wiser everyday. still having fun!