6G7A Bandmaster blonde export version '61

[tony hunt]

This looks like a keeper. Came piggy back with a '62 blonde 2x12 box loaded with JBL K series from a bass player that had it in the corner for 20 years. He said it hat sat about ten years before he bought it.

The 45217 OT is long gone, an unknown monster 4k4 to 4 Ohm is on duty now.

There is strange tag board between the PT and main board. I seem to recall seeing something like that on an export Fender before, but may have dreamt it. Anyone seen that thing before?

I can live with the few Wima and Beyschlag replacement parts, but have this to do list:

• 1. Mains filters are shot, electrolytic goo in the dog house
  2. Sort the mains cable - chassis ground
  3. Sort out the ratty adjustabe bias mod
  4. Why was a 73k added from the Intensity ccw lug to ground?
  5. Check the Astrons for leakage, but usually they are fine.

At the moment I have a ton of day-work, so progress could be slow, but it does not look like too much to do.

Cheers, tony

[Bananafist]

I've seen that tagstrip you mention on early 60s export amps. Suggest you join the Fender brownface amp appreciation group on FB. Lots of very knowledgeable guys there specialising in brownface amps. I'm not nearly experienced enough to comment.

[rp]

I love it when you guys find lovely old stuff, I can practically smell the dusty goodness from here. I restore vicariously through you.

I bet the bass player swapped out the OT intentionally, and bet that Brit beast sounds great. Maybe he has the original? But most likely some tech kept it. You are lucky could have been all unreliable yellows astrons but you got the blue ajax.

You are so close to a cheap and easy total restore down to the last AB resistor or old cap, and seeing you have the orig 2x12 with the JBLs, I'd just set it 110% right. It'll take a long time to find a '62 OT but there's no difference to the later and here's one on ebay. If I was rich I'd buy it and send to you as a gift, as penitence for my amp crimes, but alas:

http://www.ebay.com/itm/Vintage-Fender- ... 3a9fc489ff

Not sure anymore how the Fender trannie numbers go but a BF might be indistinguishable as a replacement, dull up those solder joints! BF/SF BM OTs show up fairly often as BMs benefit from an OT upgrade. If you really must have more beef the Allen TO40MT drop in would be nice and not look so grafted. But get an orig and keep it with it anyway. It'll add much more than its cost to the value and really hasten a sale.

http://www.allenamps.com/parts.php#transformers

[Teleguy61]

The OT on ebay is from Bill at Sebago Sound.
He's a member here.

[rp]

There's a nice set of tested blue moldeds in his listings too, if I wasn't living in the land of no money I'd jump on those.

[tony hunt]

Thank you for the interst.

The OP is a while back. I am in construction in Switzerland with about 70 hour weeks. Not much free time for the amp.
This where I'm at: (writing fast on a train between tunnels)
1. Mains filters are shot, electrolytic goo in the dog house = done
2. Sort the mains cable - chassis ground = done
3. Sort out the ratty adjustabe bias mod = done
4. Why was a 73k added from the Intensity ccw lug to ground? = removed
5. Check the Astrons for leakage, but usually they are fine. = done, all good

Next post after next tunnel....

[tony hunt]

I made a false assumption and should have had my glasses on looking at that German ERO cap at the PI Entrance. Those are usually solid and don't leak DC. This one was virtually a complete DC short. Plus it was. 0,10uF, maybe a Bassplayer mod. Changed that out with a 0,01 sozo for now. I think that would have been a ceramic?

Amp came to life and is very loud with massive 3d bass.

But, oh tunnel..

[tony hunt]

(... end of tunnel, now in Germany + changed cell card in the i-thingy)

But, I cannot for the life of me get the harmonic vibrato to kick in. Trying to read up in old RCA publications etc on oscillators, but it is not sinking in to my wooden head yet.

I was able to call out a few corrections to Bob's excellent redrawn 6G7-A schematic -enclosed pdf.
Par for the course with a brown face, some parts differ here from the known schematic:
Intensity pot here is an original Fender 250K / 6G7-A Schematic = 10m-RA
Top leg voltage divider off the V3b cathode here is 4,7m / 6G7-A Schematic = 8,2m
Bias circuit is with the single cap here.

I found out that the tag board by the PT is original on export models, but has been modded with a HT fuse.

Plate and G2 voltage is a little too high for my taste right now at 475V or so. Would like to try that at 445V or so. Preamp Voltages are as per schematic, maybe it was my biasing.


Regarding the OT. These came originally with a puny tweed era Triad OT and it seems most have died and been replaced. After a lot of research I followed some recommendations from the Gretsch forum and went with the MM replacement.

RP has a good eye. That English OT monster is on the bench looking at me. It has a future!


I'm probably going to have to go on you guy's nerves with this harmonic vibrato soon.

Cheers, tony

[Stevem]

That 73k was likely to slow the pulse down more, or to get a finer range of pulse settings at one end of the intensity pots range.

[sluckey]

Page 12 of this pdf explains the operation of this vibrato circuit. It may be helpful for troubleshooting.

http://home.comcast.net/~seluckey/amps/ ... apbook.pdf

[tony hunt]

Stevem, slow is good. I'm going to look at that 73k across the intensity again. Pretty sure I noted the value wrong anyway.

Sluckey, the Scrapbook is great and and I have read that trem part before, but it was another lifetime. Thank you for the reminder.
I have good signal at the Modulator stage but ahead of that, all the lights are on, but there's just no-one at home.

[tony hunt]

Update:
The osillator kicks in now. Sounds really good. May bench is up in the attic. My girlfriend heard the vibes emanating from all around and shaking the desk. She said she thought her Mac was blowing up or something!


Summary in case anyone else delves into one of these:

• 1. I used cap and resistor decade boxes, scope and signal generator and took my time.
  
  2. In case I made a mistake, I ran the amp with the bulb limiter. Running the amp afterwards, without the limiter, resulted in the oscillator refusing to start again! (Less voltage) So I had to go through the whole procedure again without the limiter.
  
  3. The ceramic caps in the Oscillator had drifted all up to 200% of their value, so I shot gunned them to get a solid foundation to start with. Now the oscillator would start, but it really took it's time and it was very weak.
  
  4. This Bandmaster had a 250k Intensity Pot that looked original (measured 275K). I changed it out to 10M-ra as per the schematic. That seemed to help provide a wider range.
  
  5. I left the voltage divider after V3 at 4m7/470k (actually drifted to 536k) as original and not as per the schematic 8m2/470k. This is quite a difference and I need to leep an eye on it. Maybe I'm hitting V4 too hard, but the vibrato is much stronger.
  
  6. This Bandmaster has what looks like an original 2M-ra Speed pot (measured 2M4). After getting the caps in range, I found this to be the most crucial to getting the oscillator to kick in. I tried 4M as per the schematic, but the range was narrow and it would stop at low and high speeds. Going much higher than 4M meant the Oscillator would not start. With the decade box I found that a 200k in series the entrance to the orginal 2M4 Speed pot gave me the widest useful range slow to fast. Dialling this in was probably the most fun part because I could get it to go really slow.
  
  7. There is a compromise when going really slow. There is less gain at a lower oscillator frequency. Keeping the 4m7/470k volatge divider helped to counteract this.

Now it is back in the head-cab all piggy backed and the dog seems to like the vibrato. Looking foward to be able to crank it. Channel one is more powerful, I guess the vibrato drags the second channel down a bit.

I'm going to take my time and try to source some correct looking parts for the ones that I had to change out to get it running.

Thank you.