6SQ7,6SN7?

[David Root]

The one I posted is Tino Zottola's adaptation from his book (Vol. 3). It is not the original type 4.

Dave and I discussed this y'day and agreed if you drop the 20K 20W resistor you need to lower the plate voltage a lot to get it where the originals were. Look up the EH-185 circuit, that has the voltages on it and is the same as the 150, the only difference was the 185 had a much fancier cabinet.

Tino did 400V to get more power and he knew these old 6L6s can take 400V (maybe not the meal case ones). He also did the voltage divider to drop some overdrive in the mic circuit. This can be done away with if you put 270V on the plates and correspondingly lower on the other tubes. I'll probably change the PT in mine, as it is pretty clean right now but has some fairly stylish overdrive, although nothing like the original I think.

[surfsup]

I still don't understand why they put in that big resistor, aside from having a PT outputting too much voltage to begin with. (I get the plates can't handle that high a voltage, btw). My question is why use a PT generating that much V. From the schemo, unless other important things were "omitted" as well, the PS rail has all the tubes sourced in line, so all the preamp voltages will be lower than the B+. So why not just use a PT that puts out less voltage alleviating the need for this 20W resistor? I mean come on, its Gibson. I doubt they had trouble sourcing trannies.

Was it "buy in bulk" back then so they tried to use the same PT/OT for multiple amp designs and resorted to many 2nd-step processes to get voltages in the ballpark?

Is there some sort of tonal advantage to having this resistor take volts and current from the PT, a sag-effect?

[David Root]

Remember the original had a big honkin' coil/choke to make the speaker work, this was before permanent magnet speakers. This thing had a resistance of 750R, which is pretty high for a modern choke and would also drop some voltage.

Still doesn't fully explain the 20K 20W resistor. Maybe Gibson had a pile of these PTs.

[Colossal]

I've modeled the power supply with various chokes and added resistance in that field coil position. Nothing accounts for the 270V on the plates except the huge voltage drop provided by the 20k 20W resistor (plus about a 5V drop in the OT). Every other permutation results in 430-500V on the plates. FWIW, I plan to use a 10H choke (about 280R DCR) and an additional 470R 5W in series for my next build (totaling 750R to simulate the field coil's resistance). But I will be adding the 20k 20W resistor as it is key to getting the voltage down to conditions found in the original EH150s and EH185s.

My guess is that Gibson had those T-40069 PTs and it was easier to use the one-size-fits-all approach and then dump voltage as needed. What's interesting is if you take DC operating point of 270V (which is where the plates are), Vpp ends up being about 513V which is exactly the rectified voltage with a 5U4G and what you'd get using the T-40069 PT WITHOUT the 20k resistor. That 20k resistor sets the operating class to A1.

I also found an old 6L6G characteristic curve showing 270V Plates / 270V screens and a cathode bias resistor of 125R for Class A1 push-pull conditions.

[David Root]

That makes all kinds of sense. I think you got it!

[M Fowler]

Since replacing the NOS preamps tubes with Chinese tubes there was a great improvement but still have oscillation at 2pm on OD channel.

I am going to try Dave's big resistor dropping trick and maybe remove the voltage divider as well.

I used Hammond 274BX PT 760CT
Hammond 1645 OT 5K 30W
Choke Hammond 158M 10H 100mA

[gary sanders]

None of the schematics have plate voltages on them.Whats the target for the pre amp tubes?

And if anyone has a couple of 6SQ7's needing a home I'll do some trading or buy them.Two of the three I got on ebay were DOA.....

[Colossal]

None of the schematics have plate voltages on them.Whats the target for the pre amp tubes?

And if anyone has a couple of 6SQ7's needing a home I'll do some trading or buy them.Two of the three I got on ebay were DOA.....

Gary,

I have done a lot of research on the EH150 (style IV = 1940s) and I have never seen any of the preamp voltages. The PT put out 408-0-408. The schematic for the EH-185 which came later uses the same PT and output section as the earlier EH-150. The EH-185 schematic shows a plate voltage on the 6L6s at 270V and the cathode voltage at 20V with 200R bias resistor. The one I built was in Class AB. The real deal is Class A1 push-pull. If you look at the schematic for the EH150, there is a 20k 20W bleeder resistor in parallel with the second filter cap, just in front of the plates (B+1). This has to be helping to set the operating class because without it the plate voltage will be about 496-500V. So for it to have 270V on the plates, this resistor must factor in. I have parts to build a new one and some lovely old tubes to roll. I'll check my stash and see what I have for 6SQ7s.

Check your PM.

[M Fowler]

A different version of the EH-185 has voltage on it 275v?



Past threads on the Gibson EH150
https://tubeamparchive.com/viewtopic.ph ... bson+eh150

https://tubeamparchive.com/viewtopic.ph ... bson+eh150