A modified Magnavox 8802 in the Gillespie style

I wasn't sure about the 7355 transformers, I've got two quads of nos tubes, so maybe when I get a little more experience I'll try to build a copy of the A500 amp circuit.

Should turn out nice. I hear really good things about the A500. They didn't shirk on the gravity of those things, the complete amp is very heavy and the transformers are pretty stout. I have one amp running 7355's, but its not an H-K.
 
Thanks Dave! Do you think I should put a heat sink on the 337? I think there will be enough space, or is it sufficient to mount it to the chassis as long as it has good contact?

Thanks again for your help,
Dennis
 
Dave, I just want to make sure I understand, do you mean to replace the 100 ohm resistors on the screen grids to 180 ohm? It appears that the resistor between the cathodes is a 10 ohm on the schematic, I just want to make sure. Also, just to clarify, should I copy the NFB from the 9300 schematic, or the 8802? Sorry for the confusion, I'm easily confused! I just want to make sure since I'm going to use the 9300 OPT's.
 
No. The 100Ω resistor I'm referring to is a single resistor in each channel running between the cathodes of the DRIVER tube. That resistor determines the amount of NFB applied around the amplifier circuits proper, and needs to be adjusted to account for the Gm of the 6V6s. Use the NFB design of the modified 9300 schematic that retains the original OPTs.

Dave
 
OK! In recent weeks, I've referred a few people to this thread, and realized I never followed up with the final schematic. No drastic changes, just a couple of corrections for clarity. I also made a note of which Edcor power transformer would work well if someone were to scratch-build this circuit without any original Magnavox parts.

[Edit -- updated with a couple of additional notes as requested below.]

Enjoy!
 

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OK! In recent weeks, I've referred a few people to this thread, and realized I never followed up with the final schematic. No drastic changes, just a couple of corrections for clarity. I also made a note of which Edcor power transformer would work well if someone were to scratch-build this circuit without any original Magnavox parts.

Enjoy!
Thank you for the update! For ~extra~ clarification would you be able to specify which pins are being connected to the tubes in the diagrams? I have only built solid state circuits before and I don't want to risk wiring these tubes backwards and blowing something up! Also how do I know I the tubes are biased properly?

Obin :)
 
Thank you for the update! For ~extra~ clarification would you be able to specify which pins are being connected to the tubes in the diagrams? I have only built solid state circuits before and I don't want to risk wiring these tubes backwards and blowing something up! Also how do I know I the tubes are biased properly?

Obin :)

That's what I get for using the word "final" :) I just uploaded an edit on the above post with the requested clarifications, and also made a note that you only need ONE of the EFB(tm) cathode regulators (upshot -- you need to use a matched-quad of tubes because that's the adjustment granularity). As for wiring up the tubes wrong -- I always find it handy to have a copy of the tube's data sheet handy when building. Note in my schematic update, I left off the pins used for the tube heaters (because I never draw the heaters in my schematics unless some exotic configuration is necessary).

As for bias, as noted on the schematic, you want to measure the voltage drop across the 10 ohm 2W 1% resistor that sits between the 6V6 cathodes and the LM337 output terminal. The bias will be adjusted properly when you have a 1.2V drop across that resistor, which represents 120mA of current (30mA per 6V6 of combined plate and screen current). This should be at around +22V on the 6V6 cathodes (which is a bias of -22V, because the grid is pulled down to ground).
 
That's what I get for using the word "final" :) I just uploaded an edit on the above post with the requested clarifications, and also made a note that you only need ONE of the EFB(tm) cathode regulators (upshot -- you need to use a matched-quad of tubes because that's the adjustment granularity). As for wiring up the tubes wrong -- I always find it handy to have a copy of the tube's data sheet handy when building. Note in my schematic update, I left off the pins used for the tube heaters (because I never draw the heaters in my schematics unless some exotic configuration is necessary).

As for bias, as noted on the schematic, you want to measure the voltage drop across the 10 ohm 2W 1% resistor that sits between the 6V6 cathodes and the LM337 output terminal. The bias will be adjusted properly when you have a 1.2V drop across that resistor, which represents 120mA of current (30mA per 6V6 of combined plate and screen current). This should be at around +22V on the 6V6 cathodes (which is a bias of -22V, because the grid is pulled down to ground).
Thank you for that update! One last question: will this circuit work with the 6P6S Russian tubes just as well as the 6V6? The 6P6S tubes are easier for me to find.

Thanks again!

Obin
 
Thank you for that update! One last question: will this circuit work with the 6P6S Russian tubes just as well as the 6V6? The 6P6S tubes are easier for me to find.

Thanks again!

Obin

6P6S are 100% equivalent to 6V6. That is, in fact, what I ended up installing in my son's amp (the original tubes ended up just being too tired out).
 
If you have an option to get matched tubes, go for it. I don't know the 6P6S, but the EL84 equivalents are pretty varied. The ones I have are different enough that they won't even play nice in the amplifier together.
 
If you have an option to get matched tubes, go for it. I don't know the 6P6S, but the EL84 equivalents are pretty varied. The ones I have are different enough that they won't even play nice in the amplifier together.

Yes that's a good point. In my case, I had a huge batch of 6P6Ss that I could select 4 closely-matched tubes from. If you're just going to buy some random 4 tubes, you might be better off ordering a matched quad of JJ 6V6s from e.g. Eurotubes (my go-to for JJs).
 
I finally found some time to finish the amp I built for my son for Christmas. Thanks thorpej and dgillespie for all of your help! I just finished listening to it for an hour or two, I might have to give him some socks instead!
 

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Thanks! I've got a few months to enjoy it before giving it up! I've got a couple of questions though. My voltages are a little low. I basically copied thorpej's P/S except I used a 1.5H 85 Ohm choke in the PS, and did the bucking mod and I've got:
line voltage 121v
308v before choke (C1)
298v B+ (C2)
288v (C3)
246v (C4) drivers
225v (C5) buffer (added from the 9300 dg schematic with help from Dave)
Heaters 6.3v

I'm guessing my voltages are low from the bucking mod. The rest of the build copies the final 9300 front end except changing the 100 Ohm resistor on the cathodes of the driver tube to 160 Ohm (thanks Dave!), and adding a 100k attenuator. The EFB follows thorpej's. My questions are these voltages OK? Do I still set the bias to 1.2v?

I can follow a schematic, but I am still wrapping my head around all this!

Regards,
Dennis
 
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