A Little Help For Eico's HF-81

Dave, would you recommend any of these mods or have other recommendations for the Eico HF-30 power amps? They don't have the issues with the preamp front end so that DC on the filaments is not needed, but the -30 does have both the power and output trannies missing the bottom bell, I was thinking of performing a star ground because of the magnetic induction on the chassis, recommended or not? Also, since this is a cathode biased ppp amp, would there be any way to convert to IBAM or should I just go with the matched quad for each amp? Thanks!
 
Dave, would you recommend any of these mods or have other recommendations for the Eico HF-30 power amps? They don't have the issues with the preamp front end so that DC on the filaments is not needed, but the -30 does have both the power and output trannies missing the bottom bell, I was thinking of performing a star ground because of the magnetic induction on the chassis, recommended or not? Also, since this is a cathode biased ppp amp, would there be any way to convert to IBAM or should I just go with the matched quad for each amp? Thanks
 
As requested, here is the DC Balance circuit I devised for the HF-81 project of this thread. In reality however, the basic configuration could work with any of the small push-pull 6BQ5 amplifiers from Eico, and many other manufacturers for that matter. Depending on the operating voltages in the output stage, the 165 ohm resistor might require adjustment to maintain the original current flow of a given design, and of course other designs may use other values of grid return resistors than the ones shown here (330K), but the basic configuration will work just fine.

For push-pull-parallel designs like the HF-30, the design will still balance the quiescent current flow through the primary winding of the OPT (which is the important thing) as long as the two tubes on each side of the push-pull signal are treated as "one" tube. However, the design won't balance the two tubes to each other that are on each side of the signal. That would require basically a duplication of this circuit, plus the addition of third high wattage pot to achieve a perfect balance between all four tubes. Such designs are best implemented when fixed bias is used.

Davescan0001.jpg
 
As requested, here is the DC Balance circuit I devised for the HF-81 project of this thread. In reality however, the basic configuration could work with any of the small push-pull 6BQ5 amplifiers from Eico, and many other manufacturers for that matter. Depending on the operating voltages in the output stage, the 165 ohm resistor might require adjustment to maintain the original current flow of a given design, and of course other designs may use other values of grid return resistors than the ones shown here (330K), but the basic configuration will work just fine.

For push-pull-parallel designs like the HF-30, the design will still balance the quiescent current flow through the primary winding of the OPT (which is the important thing) as long as the two tubes on each side of the push-pull signal are treated as "one" tube. However, the design won't balance the two tubes to each other that are on each side of the signal. That would require basically a duplication of this circuit, plus the addition of third high wattage pot to achieve a perfect balance between all four tubes. Such designs are best implemented when fixed bias is used.

DaveView attachment 645171
thanks, Dave, your help is much appreciated! Do you think I should stay with the multiple ground points of the original build, or go with a star ground and lift all grounds to one point? Due to the nature of the power transformer, I am thinking that may be better...
 
Being that it's a basic power amplifier, your noise issues will be significantly reduced. However, the star ground will give every possible advantage to minimizing hum due to the ground system.

Dave
 
what are typical cathode bias current readings for an Eico HF-81?

I have a "dual bias testing tool" that allows me to easily measure the current for each tube in a pair (from the chassis top side)

I tried three sets of output tubes, and here are the readings I obtained:

SET #1: old Russian Reflektor 6P14P (regular)
left-rear - 36.6 mA
left-front - 33.7 mA
right-rear - 32.3 mA
right-front - 30.2 mA

SET #2: old Russian Reflektor 6P14P-ER (heavy duty, long-life)
left-rear - 38.0 mA
left-front - 37.2 mA
right-rear - 33.3 mA
right-front - 39.0 mA

SET #3: new Russian Sovtek EL84M
left-rear - 30.5 mA
left-front - 40.9 mA
right-rear - 36.0 mA
right-front - 35.0 mA

are these readings considered normal for this amp and are they OK electrically and sound-wise?... obviously, I'd prefer if they were all reading exactly the same or very close to it

I tried doing some moving around of tubes to achieve better matching, especially within each channel, but it was confusing and I wasn't able to improve things too much... sometimes the high readings moved with the tube, sometimes not... very confusing

notes:
- the spec sheet I have for Russian 6P14P tubes shows a "limited operating value" of 65 mA for cathode current
- all these tube sets were supposedly "matched quads", but I have no way of verifying that

thank you!
 

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For the HF-81, Eico specifies that with a main B+ of 340 volts (at the cathode of the rectifier tubes), the cathode current of each output tube (by way of Ohm's Law) is 38.48 mA -- as determined by two tubes drawing 12.7 volts across a 165 Ohm cathode resistor. You can use this figure then to judge the current levels of your tubes, based on the B+ level actually present in your amplifier.

Dave
 
thanks Dave

I'll still have to check the actual B+ voltage in my unit, but looks like my readings are certainly in the zone -- at least the average that is

but what about the variation I am seeing, especially between tube pairs in each channel... are those close enough?

in one case I have a pair giving 30.5 and 40.9, which does not seem very close to me

I'm not sure what even perfectly matched tubes will end up producing in an HF-81, and just how close these pairs need to be for good sound and stable operation... should they be within say 1-2 mA of each other, or is higher still acceptable?

sorry for all the questions!, but I'm fairly new to tubes... my other tube amp (Fisher X-202) has adjustable bias, so I can dial in all the tubes precisely to the same reading

thanks
 
You are correct, those two tubes are not matched well at all. In the stock HF-81 setting (no DC balance adjustment available), a "good " balance would have the current draw readings of the two tubes in a given channel reading within 10% of each other. A tight match would be within 5%.

Dave
 
You are correct, those two tubes are not matched well at all. In the stock HF-81 setting (no DC balance adjustment available), a "good " balance would have the current draw readings of the two tubes in a given channel reading within 10% of each other. A tight match would be within 5%.
Dave

is there any way of modifying an HF-81 so that the close matching of tubes is less critical, or, perhaps introducing some sort of simple adjustment capability to at least help bring tube values closer together?

thanks!
 
Just a thought-

The 6P14P-ER and the EL84M are basically exactly the same tube.

Given that, it would be interesting to see how the amp would work with the 6P14P-ER set, with right-rear tube (33.3ma) swapped out for the left-front tube from the EL84M set (40.9ma).

I'd bet that with the other three "stronger" tubes, the 40.9ma tube would slightly go down in current draw (given that it would be sharing the supply with stronger tubes- it wouldn't be the one strong tube of the bunch anymore). That could be a well-matched "set"...

Regards,
Gordon..
 
Dave, I just finished reading all of this old thread and didn't see any corrections for power transformer shield nomenclature, so I'll offer this: A Faraday shield is an internal electrostatic shield between primary and secondary windings. It's an insulated wrap of thin sheet metal that's gapped to prevent the shorted-turn effect and usually brought out for grounding on a single hookup wire. The wide continuous copper strap applied outside the core (but under the end bells) is most often called a 'Hum Belt' and works by shunting leakage flux.
 
Mike -- Thank-you so much for adding that correction. My understanding was always that an internal electrostatic shield was just that: a gaped shield between the windings with a lead brought out for grounding, while the Faraday shield was the copper strap outside the core. You are stating that with regards to transformer construction, the internal electrostatic shield and Faraday shield are in fact one and the same, with the external copper strap flux shunt being called a Hum Belt. Makes perfect sense. I've seen the term Faraday Shield used pretty loosely with regards to transformer construction, so I always appreciate when detail accuracy is brought to light.

With regards to this thread then, and now accurately stated, the HF-81 power transformer does include an internal electrostatic Faraday Shield, but lacks the external copper strap Hum Belt -- so necessary to minimize hum when a power transformer of the size as used in the HF-81 is combined with high gain audio circuits on a rather small steel chassis.

Thanks again Mike for clarifying the power transformer shield nomenclature for myself, and all who champion accurate information on AK!

Dave
 
I've been reviewing the HF-81 schematic and stumbled across those small-value bypass caps at the tone amp cathodes again. Has the reason for such massive high-frequency boost in those stages been discussed in another thread? Is the net frequency response flat, and if so, how does it get there?
 
These caps are used in Eico, Fisher, and other designs. They do in fact work to produce a net flat response by working to counter Miller in the stage, whose effects go largely unchecked due to the relatively high grid impedance of the active tone control designs they are used in.

Dave
 
help, my HF-81 suddenly sounds a bit veiled, and seems to have lost its sparkle

I have a super nice HF-81 that was ground-up restored (by a pro restorer), every single wire and connection redone, new tube sockets, new Heyboer transformer, Russian PIO caps, etc., etc.... it has some very decent tubes (Mullard, etc.)

I was playing it happily for several months, logging approx.100 hours on it... from what I remember, it sounded really great

then I instead started playing my Fisher X-202 amp for a couple of months solid, which I really liked

BUT when I recently switched back to the Eico, it sounded very dull in comparison, surprisingly so

I find I have to set the treble to 2 o'clock to get a realistic level, eg. to hear cymbals and other high-frequency details... I seem to remember running flat treble, with maybe only a few degrees of treble boost needed

I must mention that I used to run "plain" Russian 6P14P output tubes, and I just now swapped in a set of Russian 6P14P-ER heavy duty tubes which I understand should sound even better (and they sound great in my Fisher!)

yes I changed output tubes, but even switching back to the 6P14P didn't seem to make a big difference: I still seem to have a fairly dull high end

my ear tells me that something must have changed but I don't really see what, or how that's possible... I did not change or adjust anything else during the hiatus, and the unit sat safely and untouched in its box

Q: any ideas of what might have happened our what I should check?

thanks!

(PS: so I took the Eico back out, and when I put the Fisher back in, WOW what a difference... clean clear sparkling high end and upper mids... crisp clear and alive... not even close)
 
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