Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bhill on March 29, 2018, 10:20:48 am
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Hi folks,
First off, I do not do this for a living. I made my living doing other electronics for many decades. But it is fun, and fixing amps is why I got into electronics in the first place. Just took me 45 years to get back to it.
Currently on the bench out in the shop is a 63 Showman that I went through and did the standard filter and cathode bypass EC replacement. The power supply for this amp seems to be a transition between the 6G setup shown on the Fender plan and the AB763, as the plate supply for the power tubes is (or was ) the series 70uf/350v EC's that the later amps used rather than the parallel 20uf/600v used in the older circuit. It also has fixed bias rather than the later adjustable bias, so it seems to be half way between the two circuits. The inside of the filter cap cover has the only chassis date stamp I can find (Aug '63), and it is missing the tube chart. These also have the presence control controlling nfb, if I am reading the plan right.
The output transformer has been changed from the specified 45550 8ohm to a Dual Showman 22889 4ohm. From the transformer wiring (pvc) it appears to be later than the cloth covered power transformer, but the date codes on both transformers (if I am reading them right) shows late 50's manufacture for both units.
When I powered it back up after the EC replacement, it has a bit of 120 hz hum. Now first off, the power tubes that came with it are a set of Ruby's of unknown parentage, let alone any sense of balance, so I suspect I have found the source of hum. I am checking with the owner to see if he has a balanced quad, but we will see. All I have on hand are two JJ half quads (spares for my bf bandmasters) so I plan to split them 1-4 and 2-3 to test that bit.
Now, given that this amp looks like a bit of a mix,
1: what modification would you recommend to the owner to properly bias
2: how or where would you mount bits and pieces so as to not foul up this old iron?
I know some folks will be horrified about modifing this amp, but if you are fighting noise all the time ???
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I would replace the 56K resistor on the rectifier/bias board with a 50K pot connected in series with a 24K (or 22K) resistor. Use a small trim pot and everything will fit on the board and can easily be returned to original if desired. Also, disconnect pin 8 of a 6L6 socket from chassis ground and install a 1Ω 1% 1W resistor between pin 8 and chassis ground. Do this for all 4 6L6 sockets. These resistors will allow you to quickly measure the cathode current of each tube to calculate static dissipation during biasing.
Look at this '57 Harvard that I did for ideas how to exactly incorporate into your Showman. Compare the original Harvard schematic/layout to the Visio schematic/layout.
http://sluckeyamps.com/harvard/harvard_5f10_schem.pdf
PS... Did you also replace the bias caps?
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Thank you sir,
About to head down to the shop to see what I can do.
Just got back from town, the owner does want to keep things as close to stock as we can even though I did go over the biasing problem. If we can keep things from being too discordant. The positive side of things is there is no time constraint, so I am not in any rush to modify his amp.
Yes, the 8/150 bias caps were replaced. I've got -54vdc (plan calls for -55). I will look through my potentiometer parts (I would have said my pot stash, but I do live in southern colo, and that could be misconstrued :icon_biggrin:)
However, I just might take my 67 Bandmaster and show him how minimal the 1 ohm on the tube socket would appear. I've been meaning to do that mod for years now, and it will give me another project for this afternoon. (escaping honeydoos)