68 Silverface Bassman rebuild

[TWANG]

It hummed and it shook. Literally shook the chassis when it was running.
And terrible crackling noises if you pushed the vol. even mildly.


I dumped the orginal boards, under the cap can, and the main board.
Replaced with combination turret/eyelet on Bakelite, w. standoffs instead
of the insulator board underneath design.

Those boards tend to attract moisture, and since this was old, and really crappy looking.. as if someone has spilled on it.
I just started from where I knew there could only be problems I made for myself.

many of the Carbon Comp resistors fell in half. Almost no pressure on them as I removed them from the board. I would say they had probably been
heated up HOT, since in my experience that can make them brittle.

I dumped almost everything from the board. Still have a couple to go.
New input and tone components.
New also over the tube sockets.
All new Filter caps and resistors.

The amp had been modified previously to me.
So it has the three prong power cord.
But though it was labelled AB165, the board really looked more like AA165.
So I made the AA amp. Which is better than the AB165 anyway, supposedly.


Part of the original horrible crackle came back.
I found that to be the Bass. Ch. vol. pot.
I replaced that with a full size Alpha, and changed the deep switch to a bright switch.
I used 200pF Silver Mica. too much!
But, messing around with it now the noise is gone, I'm going to go with 47pF, rather than the 120pF the Norm ch. uses.

the Norm ch. now has a mid pot, located where the old ground lift was.
I am not happy with my value choices, but am out of 10K pots.
But, I'll keep this in and was surprised it wasn't noisy.

About the only original components now are a couple things on the board, and the diodes.
I can't seem to find what values the diodes are. I would guess UF4007.

I want more out of the Bass Ch. but so far haven't settled on anything other than dumping the first .1uF cap, and adding the bright switch.
There's half a triode sitting there doing nothing and I'd like to get that useful.

The voltages are very close to spec.
the tubes are old GE pwr and all Mullard in the preamp.
I switched the pwr tubes to a matched pair of my Russian tubes for now.

I know there's a mod for the bias but I'll see what the voltages tell me first.

Right now, it's pretty quiet.. despite the shitty solid core wire. .many of which snap when even gently bent.. and despite the crappy way the wires are laid out.
I'll replace all the wires and lay them out correctly.
Two of the PT bolts are used for ground connects. I don't like that.
I'll drill two holes and add bolts.

The Norm. ch is actually pretty nice. breaks up ok.. still a tad late, maybe?
the bass ch. is cleany cleanerson and the cleaneriffics until you get danged loud.

So. anyone familiar/experienced with the AA165 Bassman.. chime in with suggestions.
schematics/layouts for mods appreciated especially. Nice to have in my
Bassman files, printed out!

[ToneMerc]

If I was going to gut and modify a Bassman to any particular Fender circuit, it would be a 6G6B and never look back. That circuit is a sleeper.

TM

[Timvox1]

If I was going to gut and modify a Bassman to any particular Fender circuit, it would be a 6G6B and never look back. That circuit is a sleeper.

TM

That's what I did to my 67 Silver face Bassman and I absolutely love it.
See thread below.
https://tubeamparchive.com/viewtopic.php?t=20817

[TWANG]

trying to get it biased.
the GE power tubes match at the plates.
but the 1R1% bias resistors wont match at all.
ones 48.3 the other's 38.8.
I'm gonna put my Russians back in.
They're new and supposed to be matched.

still checking out the urls! ty!

[Structo]

I'm not sure what causes that.

On my ODS build there is one socket that always seems to be a bit lower on bias current.
I've checked the bias resistors, grounds, screen resistors, solder joints, etc.

I said, what the hell it sounds good, put it back in the box.

[TWANG]

I put in my Russian 6L6GC and they are only 1v different.
I'm going to reduce my B+ though as I'm 15V over spec.
100R 10W off the standby ought to drop a bit.
I'll test that and see if I can get back to 450V plate/screen.

Norm channel is very nice. Bass ch. I still have to play with.

oh, at -46v the amp hums a bit. (sitting right in front with and level with
the speaker)
With my V that high, I am told, I should go lower. maybe -40.
But I want to drop plate/screen first.

I also think more than one pot is bad. I replaced only one, bass vol. which dumped a crackle it had.
Well.. I'll start blathering on and bore you, I need to get it back on the bench and then report.

[ToneMerc]

100R 10W off the standby ought to drop a bit.

Surely you don't mean hanging a 100R 10W to ground from the standby switch .........good luck!

TM

[cbass]

I'm not sure what causes that.

On my ODS build there is one socket that always seems to be a bit lower on bias current.
I've checked the bias resistors, grounds, screen resistors, solder joints, etc.

I said, what the hell it sounds good, put it back in the box.

I think that's because OT primaries are never perfectly matched.one side is always gonna pull a little more current even if you swap tubes around.
I use this to try and match tubes better.put the hot tube in the cold socket

[rock_mumbles]

I put in my Russian 6L6GC and they are only 1v different.
I'm going to reduce my B+ though as I'm 15V over spec.
100R 10W off the standby ought to drop a bit.
I'll test that and see if I can get back to 450V plate/screen.

Norm channel is very nice. Bass ch. I still have to play with.

oh, at -46v the amp hums a bit. (sitting right in front with and level with
the speaker)
With my V that high, I am told, I should go lower. maybe -40.
But I want to drop plate/screen first.

I also think more than one pot is bad. I replaced only one, bass vol. which dumped a crackle it had.
Well.. I'll start blathering on and bore you, I need to get it back on the bench and then report.

Don't confuse the negative bias voltage with the mV measured across the 1 ohm resistors ...

If you change the bias voltage from -46V to -40V you'll be running the tubes even hotter ...
If you change the bias so the mV measured across the 1 ohm resistors goes from 46mV to 40mV that will be running the tubes colder.

[TWANG]

no, from sby to the board.
should drop it a few volts.
I do that in my18W builds.. works there.

Also, if you don't mind, if you have something to say, even criticism, saying it outright is forum friendly. Intimating an error with no explanation is just posing.
I'm happy to be corrected.

Will be experimenting with three GE pwr tubes and my russians.
the Russians are matched.
The others are just old.

[TWANG]

Don't confuse the negative bias voltage with the mV measured across the 1 ohm resistors ...

If you change the bias voltage from -46V to -40V you'll be running the tubes even hotter ...
If you change the bias so the mV measured across the 1 ohm resistors goes from 46mV to 40mV that will be running the tubes colder.[/quote]

ah thanks!
I was going to go to -40 then test the 12Rs.
I wanted to try cooler side..

[ToneMerc]

no, from sby to the board.
should drop it a few volts.
I do that in my18W builds.. works there.

Also, if you don't mind, if you have something to say, even criticism, saying it outright is forum friendly. Intimating an error with no explanation is just posing.
I'm happy to be corrected.

Now that you have clarified yourself, carry on and point taken.

TM

[TWANG]

thanks merc!

[Firestorm]

trying to get it biased.
the GE power tubes match at the plates.
but the 1R1% bias resistors wont match at all.
ones 48.3 the other's 38.8.
I'm gonna put my Russians back in.
They're new and supposed to be matched.

still checking out the urls! ty!

The current a tube pulls is unpredictable owing to tiny variances in the placement of internal elements (especially screen grids). These things were largely assembled by hand, so it goes without saying. After they were evacuated and flashed, they could see what they'd built. The old school manufacturers did a good job of trashing tubes that didn't meet spec; less true today.

Groove Tubes did a good job matching tubes, but they sold to Fender who have their own troubles so who knows. Some vendors match tubes on old gear like Hikock that runs the plates at 200VDC or less. Tubes that match at 200 will not necessarily match at 400+ (hardly ever).

So the only choices are to buy tubes from a guy that burns them in and matches at real-world voltages; buy a lot of tubes and match them yourself in the amp; or provide a separate bias adjustment for each tube so you can make mismatched tubes match. They don't have to be perfect, 5mA difference is fine, considering the OT turns won't match perfectly either.

[TWANG]

I have the bias Rs set for 39.4 and 41.3.. pretty close!
that dropped my plate and screens to 454/453 respectively.
More cleans... I think it sound better.
I couldn't get this close using the GEs.

I have the Russian tubes in.. which are closer to 5881, and I have another set, supposed to be matched that are 6L6GC, so I'll be trying those and I have another pair, too.

I did try a new 12AT7.. brit mullard..
and it tests better than my RCA which is microphonic and his brit mullard
lit up really bright. not red plating but just xmas tree inside.

I will probably try AX and AY 7s' in the AT7 spot.

I like the bass ch. better now than anything so far. but it seems
still .. something lacking.
I think it's time to play it for a couple hours and get to really hear the thing.