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12.10.199.11
In Reply to: RE: I'm a sucker for a pretty face........ But I need biasing help posted by Palustris on November 10, 2014 at 09:55:58
A single bias adjustment strongly suggests the need for a TIGHTLY matched quad of O/P tubes. The 6GW8/ECL86 is quite scarce and sourcing a nice, matched, quad is likely to be quite difficult. In spite of reduced power O/P, switching to the 6BM8/ECL82 may be best.
The linked schematic shows an AC heated phono section. IMO/IME, the only tube to use, without constructing a DC heater supply, is the Sovtek 12AX7LPS. The 'LPS is a genuine 7025 equivalent that contains a spiral wound, hum bucking, heater.
Scan the archives for my many posts regarding source selection upgrades.
An entirely different refurbishment tack is conversion to a "straight up and down" power amplifier that uses EL84/6BQ5 O/P tubes run conservatively. I would switch to SS rectified B+, if this option is selected. A somewhat "taller" B+ rail would allow, among several things, combination biased "finals".
Eli D.
Follow Ups:
Eli, Are you saying that a set of 6bm8/ECL82's would be a drop in replacement in this amp but at a reduced power output? After I bought this I did a little homework ( I know,assbackwards ) and discovered the Ecl86 is kind of scarce but I happen to get a sleeve of 5 Amperex on ebay a couple of days ago. Of course they're not matched but should help if they disappear entirerly.
PaulB
The 6BM8/ECL82 is not a drop in replacement. Parts value changes are needed. The exercise has been done more than once. Too bad a Russian 6GW8 equivalent is not available.
Send that sleeve of 5 6GW8s to Jim McShane and have him cull out the 2 best pairs. Combination bias will bring things into line. While you're at it, send Jim the tubes that came with the unit for check out too.
Eli D.
Eli,
Do yo think that perhaps a modification to a dual pot bias setup, or perhaps modifying the biasing in some other way to accommodate less well-matched tubes would be useful? Not really suggesting he go drill holes in the chassis, but a trimmer style pot could be mounted inside, possibly even under an existing vent hole to allow adjustment with a tweaker...
Exactly the approach I would take. Really, you just have to set it up once wiht the amp open and then check it after 10-20 hours and then tubes should be pretty darn stable. Then maybe once a year you open it up and check the bias across whatever cathode resistors you have. Otherwise you would have to install an extra external test point or a switch to choose which channel. I would also install an extra pot to allow balancing the tubes on each channel as in the later revision. I am sure it can fit in there somewhere. Then I would just do it inside the amp once a year or so....
Hey Sherwood, I like that idea....convert it so each otuput tube can be biased seperately. Don, Mikey, Eli, anyone. Could this be done in a practical way?
PaulB
Hi,I just looked at the schematic I found here: http://www.hhscott.com/pdf/fs/200-C1.JPG
It shows that this amp has a bias pot for left and for right channels. It also shows two bias test points, and a bias selector switch.
I suspect that the one you show in the picture with the cemented knob is in fact an overall bias voltage adjust, and that there are two more bias pots, one for each channel somewhere else on the amp, probably near the power tubes. With that kind of setup, you actually have a great deal of flexibility already with regards to balancing your channels, and even biasing up or down overall, to accommodate weak, strong, or different tube types. I think you're in good shape. That overall adjust should even allow you to compensate for the increased voltage when you replace the original diodes and/or caps and wind up with increased bias voltage. That's pretty cool actually, I had to make my own adjust circuit on my Sherwood S-5000 when I replace the selenium rectifier.
Edits: 11/10/14 11/10/14
Oh my gosh, UPDATE.... I was just looking at the picture of the deck in my post and right there between each pair of outpit tubes is a pot marked "bal". How in the world did I miss those. So, there is 1-bias pot, a test point, a hum left , a hum right, and 2-BALANCE POTS, 1 between each pair OP tubes. With this in mind What would be my balancing proceedure?
PaulB
Look to see how they are connected. Good chance they are in the cathode circuit, with the center wiper towards ground. If so, you can sometimes hear tube balance by setting for minimum hum. Do this with all controls turned down. Start with the balance pot centered.
(unlike hum balance controls for the preamp, which should be set with the controls turned up)
Well, good news and bad news... Good news is you have a flexible bias setup. Bad news is that schematic I was looking at must be a different revision. It does not show the overall bias voltage adjust (assuming that's what it is) and while it does have L and R bias adjusts, does not call them 'BAL.' I'm thinking this is a different revision, so without sleuthing around inside I can only hazard a guess. Usually in bias balance setups, like in an S-5000, there is a pot that has each side tied to one tube in the pair, and the wiper leg goes to the supply. In this configuration, you adjust for a center value until each tube is drawing the same current. Then you do the same for the other pair. You never get all 4 pulling the same current unless they are well matched, but you do get even loading of the tubes in each channel.
Unless there is some other rev. schematic out there (look around, SAMs too) I'll need to see more...
ok Tomorrow, if I get a chance, I'll pop the bottom off and post a pic. Maybe then we can get a handle on this.
Thanks Sherwood and all who responded. PaulB
SS rectify the B+ and "stand" each O/P tube pair on a 470 Ω/100 μF. RC bias network. Now, you have a convenient "idle" current test point and only 1 trim pot./channel is needed. Close matching of gm is ALWAYS necessary, when selecting PP O/P tube pairs. Combination bias, as described, allows for minor differences in cathode current to be tolerated.
BTW, freeing up the 2 A./5 VAC winding makes adding a DC heater supply for the phono section pretty darned easy. :> D
Eli D.
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