Looking for the EH-150 Schematic out of Tino Zottola's book

[David Root]

This is Volume 3, Building Guitar & Bass Amplifiers. (Orange covers, paperback). I have a copy of it but I'll be damned if I can find it.

If anyone has a copy I would be indebted if you would be so kind as to scan the EH-150 schematic out of it and PM it to me. He made a couple of changes to up the power to 30W which I liked and can't recall all of them.

[M Fowler]

Here is the schematic plus the original design with field coil.

[David Root]

Woooo! Thanx Mark, I really appreciate that!! If you're ever in Vancouver BC contact me I will buy you lunch!

That is a nice build, too. Looks like Hammond iron. I bought an Allen amps PT-25 and an Allen Amps TO-35MT OT, and a Hammond 158Q choke. I'm doing mine in a BF Princeton Reverb chassis I got from RJ. It's aluminum but blank so I have a bit of work.

What speaker/s do you find the circuit works best with? I would guess a vintage P12N, but a bit pricey, no?

[Colossal]

David,

I built this circuit for a client and we used a Vintage 30 of all things. It sounded fantastic. My voltages were higher and I biased around 800R for 42mA which agreed with 70% of max dissipation for 6L6s. I calculated about 21W of power. Not a loud amp, but comfortably loud and the tone was amazing. We used all NOS for the preamp tubes.

Regards,
Dave

[M Fowler]

Your amp looks so sweet Dave love that retro look.

[Colossal]

Your amp looks so sweet Dave love that retro look.

Thanks Mark, it was good collaborating with you on that build. I'm looking forward to the next one.

[David Root]

Colossal, that looks superb! Vintage 30, I would never have expected that.

Would you think a Celestion Gold or an Alnico Tone Tubby would perform?

I'm doing all NOS too, all glass. I have a Marconi CV1991, an RCA and a Sylvania silvertop 6SQ7GT. 6L6s are a pair of WWII Sylvania JAN CHS GAs. '40s National Union Sound Exxtra gray plate 6N7, choice of two.

Slight detour, I'm planning on using an RCA tan base '61 5R4GYB rectifier, as the PT is only 2A on the 5V. How would you rate that vs. the spec'd 5U4G?

[Colossal]

David,

I bet a Gold would sound really, really good. I do plan to try a Blue with the next one I build. The V30 was amazing though. It was very focused but not excessively detailed, had a great midrange sort of honk and the low end was tight and punchy. Highs were very smooth and not at all tinny. The player uses his own hand made maple archtops with a PAF or Charlie Christian pickup in the neck position. The very definition of jazz tone. As Mark Fowler noted, the amp is on the darker side (which I like) and the V30 seemed to be a good compliment to it and the result was just very even sounding; not too much of any one thing. The breakup was just perfect.

As for a 5R4, I don't see why not. You might get a bit more voltage drop which will be fine and pull less PT current as you have noted. Your tube selection sounds really nice.

Regards,
Dave

[M Fowler]

I just replaced all my NOS 6SQ7 tubes with new production that took out the noise and instability on the OD channel.

Adjusting the voltage divider wasn't working oscillation at about 2 pm on the OD volume knob but the new tube solved that problem.

I have a new 6N7P tube comiing to see if that works to replace the NOS 6N7.

Great sounding amps for jazz or blues. Not sure the OD channel is even needed.

[M Fowler]

I do not have a dedicated speaker for this amp head. We have used the Z blue/gold 212 cab but I am sure we used the Epiphone Lady Luck speaker last time.

I like the Vintage 30 because in a dark amp like the EH150 it works well.
I would love to try out those tone tubby speakers though.

[David Root]

Both the original and TZ's modified circuit operate in class A1. In A2 it should get 21 x 1.4142 = 30W more or less. It may have moved into AB1 by then though, don't you think?

Has anyone tried that? I expect the cathode bypass cap to move it towards fixed bias tonally.

Could put it on a switch!

[David Root]

Mark, are you aware that later in the 6SQ7GT production cycle there was a look-alike (not exactly a fake) made by putting 7-pin 6AV6 guts in an octal package.? "Electrically equivalent", no doubt, but sonically equivalent? I doubt it.

Four years ago when I started looking for 6SQ7GTs for this amp, I quickly discovered that tube vendors did not distinguish between the two and I was paying full price for these cuckoo's eggs in an octal nest. I posted my misadventures here in a thread called Will the Real 6SQ7GT Please Stand Up?

I'm curious, which type were the ones you replaced?

[M Fowler]

David I had a mixture of metal and glass 6SQ7 not sure if the glass ones are the fake ones!

I can't see how the EH150 can be class A operation the voltage is high and puts out 30w. Collosal and I have been discussing that the past two days

Mark

[Colossal]

Both the original and TZ's modified circuit operate in class A1. In A2 it should get 21 x 1.4142 = 30W more or less. It may have moved into AB1 by then though, don't you think?

David,

I have researched the various EH150 incarnations and I've seen them listed as Class A 15W. Now I'm wondering if that's the Type I where Gibson used an interstage transformer and a pair of 6N6s for output tubes. The Type II/IV model where they switched to 6L6s and a 5U4G in 1941-42 seems to be Class AB. The original (Type III/IV) operated at >490V on the 6L6 plates <i>and</i> screens and was biased with 200R! That's cookin' at greater than 70% dissipation so I'm sure the tubes are drawing current most of the cycle but that's only one of the tenets of Class A. I biased my build at 70% and the cathode resistor was 800-850R (I used a variable bias setup so I could dial in the desired current), 42mA of current with 500V on the plates. I lowered the screen voltages significantly however. I could kick myself for not measuring the grid voltage at idle

So, yeah, I'm wondering if the Class A moniker came from the very first EH150s and just carried over into internet lore. I don't think that's intentional though like today's marketing hype over Class in AC30s, but just historical bits fading in time. Relatively few even know about those old amps anymore except the purists and builders.

[David Root]

Gents, good points, I had not reviewed the plate voltages. Basically above about 350 Vp they go into AB1, right? Like the AC30.

Even so, adding that cathode bypass cap should increase power too.