Can someone explain this ell80 tube schematic?

[Carbsandcaps]

Working on a fisher 49a amplifier that I’m rehabbing to be a standalone amplifier. It uses a tube/ transistor hybrid design. It uses an ell80 tube to feed the transformer to the transistors. I thought the ell80 was a standalone push/pull output tube? Seems like it’s just tied together and single output through the cathode. Then there’s a 20uf 250v capacitor after that (c7) THEN a small 16uf 16v capacitor before the transformer.

My question is, why do you need the big 20uf capacitor if it’s going into a transformer which will eliminate the dc voltage anyway? Also I have a 47uf 250v film capacitor I could put in there since it’s in the signal path as an experiment.

 

[larryderouin]

Fisher used the ELL 80 as a driver. output is coming off the common cathode. @fred soop is the MASTER HYBRID Guru on FISHERS. 2 notes
1.) Germanium output transistors.

2.) Does it still have the 4 bias pots, or were they removed for 1n2326 RCA Diodes. One of the Custom Electra VIII Service manuals (of 3) has the upgrade/mod noted in it. It was done by dealers repair shops in an attempt to stabilize Bias. If it has pots, DO NOT TOUCH THEM. They had a tendency to go open and blow the transistors. Fred has a bias circuit update/upgrade that he developed for the 600-T (very similar circuit) and it's adaptable to the 49A.


Full schematic (one side) for Context.

 

[Carbsandcaps]

Fisher used the ELL 80 as a driver. output is coming off the common cathode. @fred soop is the MASTER HYBRID Guru on FISHERS. 2 notes
1.) Germanium output transistors.

2.) Does it still have the 4 bias pots, or were they removed for 1n2326 RCA Diodes. One of the Custom Electra VIII Service manuals (of 3) has the upgrade/mod noted in it. It was done by dealers repair shops in an attempt to stabilize Bias. If it has pots, DO NOT TOUCH THEM. They had a tendency to go open and blow the transistors. Fred has a bias circuit update/upgrade that he developed for the 600-T (very similar circuit) and it's adaptable to the 49A.


Full schematic (one side) for Context.
View attachment 3110919

No the pots were bypassed with the diode upgrade. It’s just half a 12ax7 feeding the ell80 I wonder why they chose that instead of another 12ax7 or 6aq5 or something simpler. Maybe an importance issue?

 

[larryderouin]

12ax7 Dual Triode being used as a Voltage amp for the ELL80's. Only need one to feed both. Keeping with FISHER using the 12ax7 heavily, probably ended up with the bean counters calling the shots to keep it as inexpensive as possible to maximize profit. My Custom Electra VIII is a bit Bass shy (8" drivers in closed cab.). My Futura VI (59A bigger brother of the 49A.) Is better power and basswise but still it won't get windows resonating to bass.

 

[Dave451]

Interesting setup!

 

[Carbsandcaps]

Well either way I put the 47uf 250v film caps in and 4 4.7uf film caps to get the 16uf for the output to the transformers, replaced all the eelctrolytics and some coupling caps. I wasn’t getting any sound until I realized I had to wire the 12ax7 heater because it uses the output to the preamp for voltage. So I hooked that up to the 6.3v tap, put in a power led and switch. It sound pretty good! Definitely a good garage amplifier.

 

[larryderouin]

Check the voltage on V201. That circuit in normal operation feeds 3 additional 12ax7's in series, it's a 44V feed. IIRC a 270Ω 5w resistor inline on that circuit would bring it down to about 11.6-12.0v. Fisher had a tendency to starve 12ax7 heaters (some circuits down to 10.6v) which contributes to less noise, and tube longevity, but warm up times are a little longer. 12.6V + or - 5% (12.0 to 13.23) is usually ok. I would stay on the low side of 12.6.

 

[gadget73]

kind of an odd way to use a dual power pentode. Its strapped so instead of two 6 watt pentodes you get one 12 watt one. Its operating as a cathode follower to drive that transformer. I think if I had one of these and the ELL80 was done for it would get converted to use some single pentode that costs less.

 

[Carbsandcaps]

Check the voltage on V201. That circuit in normal operation feeds 3 additional 12ax7's in series, it's a 44V feed. IIRC a 270Ω 5w resistor inline on that circuit would bring it down to about 11.6-12.0v. Fisher had a tendency to starve 12ax7 heaters (some circuits down to 10.6v) which contributes to less noise, and tube longevity, but warm up times are a little longer. 12.6V + or - 5% (12.0 to 13.23) is usually ok. I would stay on the low side of 12.6.

In a brain fog moment I tapped the 12ax7 heater directly to the 6.3v output from the transformer. Which would be fine except I sent it straight to pins 4 and 5 splitting the 6.3v but it still worked! I guess he cause of the 120v line input it’s closer to 7.3v so 3.5v to each heater may be enough to at least start flowing electrons.

 

[Carbsandcaps]

kind of an odd way to use a dual power pentode. Its strapped so instead of two 6 watt pentodes you get one 12 watt one. Its operating as a cathode follower to drive that transformer. I think if I had one of these and the ELL80 was done for it would get converted to use some single pentode that costs less.

That’s what I was wondering. Seems like a strange tube to use. I don’t know if they were cheaper back then but they’re pretty pricey now, almost double the price of an el84.

 

[gadget73]

also not impossible they just had the stock from the Philharmonic model that ran them. They must have had a reason for it, and price is a pretty good one.

This might be a good spot for a 6p15p tube. Its kinda like a 6bq5 but with lower screen voltage ratings. Here it would be used as a triode though so the screen limits wouldn't be an issue. They tend to be very cheap too.

possible you'd have to tinker with the 6.8k to ground if the bias ends up a little off but it shouldn't be enough to upset anything.

Something that does seem extra odd to me is the 16uf cap with the resistor across it in series with the 20uf cap. The 20uf cap makes the transformer a parallel feed, blocking the DC current flow. That much makes sense, but why two caps in series, one with a resistor across it?

 

[Carbsandcaps]

also not impossible they just had the stock from the Philharmonic model that ran them. They must have had a reason for it, and price is a pretty good one.

This might be a good spot for a 6p15p tube. Its kinda like a 6bq5 but with lower screen voltage ratings. Here it would be used as a triode though so the screen limits wouldn't be an issue. They tend to be very cheap too.

possible you'd have to tinker with the 6.8k to ground if the bias ends up a little off but it shouldn't be enough to upset anything.

Something that does seem extra odd to me is the 16uf cap with the resistor across it in series with the 20uf cap. The 20uf cap makes the transformer a parallel feed, blocking the DC current flow. That much makes sense, but why two caps in series, one with a resistor across it?

Agreed. The 20uf cap is rated at 250v, the 16uf is rated at 16v. So the 20uf takes the dc out and maybe the 16uf has something to do with tone? I just thought the transformer would take the dc out making the 20uf redundant.

 

[gadget73]

The transformer would block it from the transistors, but its probably not a type that can handle any DC current flow in the primary. Probably fine wire and no air gap so the 26ma of current the ELL80 is flowing would still be enough to saturate it.

both sections together are dissipating less than 4 watts so its not like the tube is stressed at all.

 

[Carbsandcaps]

The transformer would block it from the transistors, but its probably not a type that can handle any DC current flow in the primary. Probably fine wire and no air gap so the 26ma of current the ELL80 is flowing would still be enough to saturate it.

both sections together are dissipating less than 4 watts so its not like the tube is stressed at all.

Ah ok that makes sense. It’s a relatively small transformer.

 

[Tom Bavis]

I suspect the odd R-C pair is for low-frequency stability of the feedback loop - there are two branches, from primary and speaker output. The latter has the usual R-C for high frequency phase lead. Really not that different from feedback compensation in an all-tube circuit - there's a transformer in the loop, after all...

 

[Carbsandcaps]

Well I finally got it setup in my garage with my crown ic-150 preamp and it’s sounds …. Terrible! It has a ton of distortion from mid to high volume. Maybe that’s just how it is.

 

[larryderouin]

Actually they don't have much if anymore distortion in normal configuration with the original preamp. My Custom electra VIII and Futura VI both have the Hybrids (49A in the C-E-490 and 59A in the F-590). Only real difference between them is output transistor count and associated extra components in the 59A. Miswiring the Molex, or component mismatch, are possible causes or the Crown is overdriving it. Check the 5 fuses on it also. May have some corrosion.

 

[Carbsandcaps]

Actually they don't have much if anymore distortion in normal configuration with the original preamp. My Custom electra VIII and Futura VI both have the Hybrids (49A in the C-E-490 and 59A in the F-590). Only real difference between them is output transistor count and associated extra components in the 59A. Miswiring the Molex, or component mismatch, are possible causes or the Crown is overdriving it. Check the 5 fuses on it also. May have some corrosion.

Ya that’s what I was wondering. I’m using a Bluetooth adapter, I hooked it up right to it and it’s still distorted. Could it be an impedance difference thing since the original preamp was tube?

 

[larryderouin]

Impedance mismatch is certainly possible.

 

[gadget73]

wouldn't think thats a big issue. The only real magic trick is that the source impedance should be lower than the load impedance when doing voltage connections like this. This has input impedance like a tube amp, 1 megohm, so anything should be able to drive it with no fuss.

curious if its making rated power and clipping symmetrically. This is kind of odd being a direct coupled split rail power supply. That was normal by the mid 70s but first gen solid state often had cap coupled outputs and a single supply. If the power supply voltages aren't even but opposite polarity it will do strange things.