DIY 71A on the Brain

[marantzfan]

So far, so good this weekend getting things put together. Little by little, I’m getting there .



This is my fist time using interstage transformers. Since both the primary and secondary are 5K ohms, does it matter which way I connect it?

 

[paul_b]

I do not believe it matters. A lot of them are bifilar, so the primary and secondary are wound together (winding two wires onto the transformer at once).

 

[Wntrmute2]

How did you decide to mount those big Miflex caps?

 

[marantzfan]

How did you decide to mount those big Miflex caps?

Finally have enough assembled to illustrate the answer to this question.

Directly beneath (and using the same bolts as) the interstage transformers, I am using some brackets.

With these brackets, a pair of 18” zip ties for each capacitor to cinch them down.

 

[paul_b]

On those 22 ohm resistors that go from the pot to the socket, I would slip some insulating material over the leads. If the resistor lead touches the corner of that pot body, you'll smoke something (last time I saw this, it was the plate choke).

 

[Wntrmute2]

Envious of your work!

 

[mpholland]

Not sure how I missed this thread. Guess I should peruse DIY more often. Speaking of stainless, and for future reference, you can get stainless steel zip ties that are big enough to tie those caps down. These are 14" long and about 3/16" wide.

Stainless Zip Ties

 

[marantzfan]

Update. Everything is soldered, so I took it out to the garage for some testing. Overall voltages are close, but maybe a little out of range; particularly the 71A filament voltage which looks low. They are as follows:

71A Fil -- 4V DC (L)
71A Fil -- 4V DC (R)

71A Plate -- 233V (L)
71A Plate -- 233V (R)

6E5P Fil -- 6.8V (L)
6E5P Fil -- 6.8V (R)

6E5P Plate -- 240V (L)
6E5P Plate -- 240V(R)

Wall -- 120V

B+ -- 243V

 

[MrEd]

Nicely matched, what is the filament voltage supposed to be?

 

[marantzfan]

Nicely matched, what is the filament voltage supposed to be?

Supposed to be 5V I believe, so we’re a little low.

 

[Wntrmute2]

I believe those tube filaments draw only 250mA each. What is the capacity of the winding?

 

[paul_b]

I would get another decimal point on the 71A filament voltages if possible.

You can parallel some resistors across the 8 ohm resistor to bring the voltage to spec. If you have an extra 10 ohm 5W hum pot, you can wire that as a 10 ohm variable resistor and clip lead that across one 8 ohm resistor, then turn it down until you see 5V across the filaments.

Of course putting a 10 ohm resistor across the 8 ohm resistor may bring the voltage up past 5V, in which case you'll have to clip lead in other values until you find one that produces the desired numbers.

 

[Opa1]

Nice work! I missed this thread. I like the Thermionic Lab stuff.

 

[marantzfan]

I would get another decimal point on the 71A filament voltages if possible.

You can parallel some resistors across the 8 ohm resistor to bring the voltage to spec. If you have an extra 10 ohm 5W hum pot, you can wire that as a 10 ohm variable resistor and clip lead that across one 8 ohm resistor, then turn it down until you see 5V across the filaments.

Of course putting a 10 ohm resistor across the 8 ohm resistor may bring the voltage up past 5V, in which case you'll have to clip lead in other values until you find one that produces the desired numbers.

You know, I just so happen to have an extra 10 ohm hum pot laying around. I will get on that later today! Thanks again Paul!

 

[marantzfan]

Update: Following Paul's advice, with the pot wide open with what measured at 9.3 ohms on one pot and 9.6 ohm on the other, I was able to dial the 71A filament voltage to 5.18V.

So, am I right to assume that adding a 10R/5W resistor across each 8 ohm resistor is the way to go?

Decided to take all new voltage measurements while I was at it:

71A Fil -- 5.18V DC (L)
71A Fil -- 5.17V DC (R)

71A Plate -- 230V (L)
71A Plate -- 230V (R)

6E5P Fil -- 6.8V (L) — High?
6E5P Fil -- 6.8V (R) — High?

6E5P Plate -- 239V (L)
6E5P Plate -- 239V(R)

Wall -- 120V

B+ -- 242V

 

[paul_b]

You could take the 8 ohm resistor out and replace it with a 5 ohm 3W resistor, or put 10 ohms across what you have (3W rated should be fine).

 

[marantzfan]

You could take the 8 ohm resistor out and replace it with a 5 ohm 3W resistor, or put 10 ohms across what you have (3W rated should be fine).

Ok, I’ll do that. Any concerns with B+ voltage being a little high?

 

[paul_b]

You're about 5% high on voltages, I don't think I'd worry too much about that. You could put a CL-90 in series with the primary hot incoming power transformer lead to drop a little bit of extra voltage. You might knock that 5% down to 3%, and this will also nudge down the 6.3V winding and the 5V winding a little as well.

 

[marantzfan]

Resistors and CL-90 are arriving Saturday from Digikey. Updates to come

 

[marantzfan]

Digikey delivery was delayed a little bit but came in today. Popped in the CL-90 and resistors today on my lunch break and things are looking (and sounding) pretty good on my test setup in the garage.

71A Fil -- 5.01V DC (L)
71A Fil -- 5.00V DC (R)

71A Plate -- 223V (L)
71A Plate -- 223V (R)

6E5P Fil -- 6.7V (L) — High?
6E5P Fil -- 6.7V (R) — High?

6E5P Plate -- 229V (L)
6E5P Plate -- 229V(R)

Wall -- 120V

B+ -- 231V