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Old 5th August 2008, 08:41 PM   #1
pchw is offline pchw  United States
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Default How to figure out the bias point of fisher X-100A

I have a Fisher X-100A that is in the original form. I plan to to replace the old parts even though it is still very quiet with no audible hum from 2 feet away.

According to the (attached) service manual, the cathode of the EL84's are bias at -12V - 18V at the grid and 30V at the cathode. It is using a combination of a string of heaters (of 2 12AX7) + 22R resistor in series and parallel to a 2.2K resistor to ground. I am wonder:
1. why the cathode is at 30V but not higher,, 3 x 12AX7 in series is already at 12.6*3 = 37.8!! and these is a 22R resistor in the string. Maybe the grids are drawing some current??
2. Let say the cathode is indeed at 30V, then the 2.2K resistor at the top is drawing 13.6ma, and the heater should be drawing 150ma which implies the 4 EL84 are drawing around 40ma each. Correct? Looking at this tiny PT, I have serious doubt it can supply this much current. So, what am I missing here?

I know this is kind of dumb questions, but I just couldn't quite figure out how those voltages are derived with my minimal knowledge.

Thanks,
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Old 5th August 2008, 09:37 PM   #2
qwad is offline qwad  Australia
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Hi, l shouldnt but in here, as l am not nearly knowledgeable to answer your questions, only l have one of these too, but mine originally came with 7189 tubes[ a higher rated version of the el 84 which mine are equipped with as 7189's are very rare and if you can find them are worth an arm and a leg, though there are some russian versions about, at much more reasonable prices] having said that; l upgraded some of the components with minimal sonic differences, caps mainly, so my advice is if you have nothing major wrong with your amp, leave it alone and enjoy it as it is....
cheers T.C.
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Old 5th August 2008, 09:52 PM   #3
pchw is offline pchw  United States
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I do have a quad of NOS RCA 7189, but for now, it is cheap Sovtek time :-) My amp is still amazingly quiet but doesn't sound as nice the Eico HF-81. My questions are kind of for my learning to understand how this cathode bias work. Since I have a 12.6VAC around, I can mount it to the chassis and run all 12AX7 off this little toroid, but then I will need to know what to put at the cathode of the output tubes
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Old 5th August 2008, 11:23 PM   #4
qwad is offline qwad  Australia
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Yes, it is true the eico's by all accounts were a sweet sounding amp. this all comes down to sonic signatures of the various components used in the original design; o/p transformers etc, sorry l cant be more specific, though l think your sovteks are as good as anything, l wouldnt worry about putting in the 7189's as you may not notice a real big difference, though it may not hurt to try 'em out.... as for the rest of your questions it wouldnt hurt to ask them on the vintage page of audio asylum there may be more knowledgable people there to answer your questions.........
good luck and cheers T.C.
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Old 6th August 2008, 08:08 AM   #5
qwad is offline qwad  Australia
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should've waited for you to post a reply, but l was chewing over this all morning when l realised what is causing such a sonic difference between the amps, when it occured to me that if my memory serves me right, the eico o/p transformer has an ul tap, so the output tubes are connected in ultra linear mode, wheras the fisher hasnt, the other thing you can try to sweeten the fisher up is to connect the op tubes in triode mode, [with a slight loss of power] if that doesnt make you happy you can try a svetlana sv83 tube in place of the 6bq5's which are similar [ AND ACCORDING TO REPORTS NICE SOUNDING TUBE oooops sorry didnt mean to shout], only the pins are configured differently, so a little rewiring will be necessary............
there now l got it off my chest, and mind, l racked my brains for that tubes number[ the sv 83 that is] cheers T.C.

ps; decware builds and sells amps made with this tube, and as far as l can ascertain you should be able to get them from triode electronics................ all the best and good luck
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Old 6th August 2008, 05:13 PM   #6
pchw is offline pchw  United States
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The Eico HF-81 is also pentode only, no UL tap. It is based on Williamson design as well but uses tube rectification instead of SS diodes. I think the difference in the OT and the preamp section probably the major reason to explain the tonal difference. Quite frankly, which one sounds better is a personal opinion, not a universal one :-) BTW, I also have a Fisher 400 receiver, but I haven't decided whether I shall gut the irons or restore it - the 7868 tubes are a little hard to get by these days, and I don't really need to FM section .....
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Old 6th August 2008, 09:53 PM   #7
qwad is offline qwad  Australia
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Fred, yes there is something to say for tube rectification, according to lore l have seen on the net this could account for the sonic differences that you have noticed, those that build their own from scratch would not consider anything else......... l remember reading in "sound practices" that l used to subscribe to; so l guess that explains it, should'ave thought of it and asked...... lucky guy having a 400, l have never seen one, doubt if any made it down here.............
cheers T.C.
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Old 8th August 2008, 04:39 AM   #8
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Fred,
Don't get too hung up on the figures on the schematic.

These figures reflect that they are running the heaters of the V1 V2 and V3 low, => at 9V instead of 12V.

That was to get the current down a bit. Ideal current in that heater string (for a full 12V across each heater) is 150mA as you stated. That would infer an idle current of 150/4 = 37.5 mA per output tube. With an anode voltage of 390V that is 14.6 Watts dissipation which is over the rating of these tubes.

So what they have done is drop the current down. The effective bias is set by the voltage drop across V3 heater and the 22 Ohm resistor (adjust that 22 Ohm to get bias currents right).
A bit of back calculation suggests 3V across the 22 Ohm => 136mA or 34mA per tube. That is still running the output tubes pretty hard and a matched set of output tubes would be essential.

They have accepted the compromise of preamp tube heaters running low to get output tube dissipation down a bit. Of-course anyone designing this today and posting it for discussion would get an absolute caning. This is design "down to a price" and not "up to a standard" at its worst. There was a whole rash of amps which came out around the same time where the designers thought they were being "Clever" to use preamp heaters as cathode bias resistors in the power tubes. It gave a "free" heater supply so the power tranny could be smaller and cheaper and the amp could be $2 cheaper.

You know the old expression "He was so sharp its a wonder he did'nt cut himself". Well suffice to say that at the time there were some Hari-Kari types about designing amplifiers or at least one's who should have but did'nt, tell the accountant to "take a hike" (preferably off something tall).

On the positive side I guess I do need to conceed that they saved a whole 5 watts of power and heat.

I even once saw someone sketch up an amp design where he was going to use the output tube heaters themselves as their own cathode bias resistors. Boy did I have a laugh, no heater means no current means no heater etc. - you get the drift.

Cheers,
Ian
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Old 8th August 2008, 05:33 AM   #9
pchw is offline pchw  United States
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Hi Ian,

I really appreciate your detail explanation. I was scratching my head to try figuring
the voltage as indicated in the service manual :-) One more question if you don't mind. There is a 2.2K resistor in parallel to the heater strings. What is the usage of it?

Thanks!!
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Old 8th August 2008, 05:50 AM   #10
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Fred,

I have to admit I don't know.
I looked at it to see if it was giving some protection in case of an open circuit heater (or pulled tube) on V1, V2 or V3 but it does'nt provide that.

Only thing it does do is keep the output stage "ticking" at very low idle currents if the above occurs and stops the cathodes from "floating".

Unless I'm missing something obvious I don't think it does anything except add about 3 and a bit mA per output tube.

Anyone else have a clue?

Cheers,
Ian
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