1965 blackface deluxe reverb, needs help.

[mikep]

Hey Guys,

I'm checking back with an amp that you all helped me to fix a couple months ago.
At the time, there was a broken ground wire, which I fixed and was able to play the amp.
At the time it was all original. It took me a while, and courage, to get back into it to change the
2 prong and all the caps.

The amp has much improved headroom and is also much louder, however, I just feel that it is
a little bright and ice picky particularly on the 2nd channel. Something tells me its not as good
as it can be. Its lacking the sweetness that I would suspect this amp should have.
I'm hoping you guys might have some recommendations.

Here's the update and measurements:

I changed all the power filter, bias, and cathode caps to new F+T in the power section and
nichilcon for bias and cathode. The death cap was removed and a 3 prong installed.


At the doghouse cap cans I get: 412,412,408,314,245. respectively. Bias cap: -49
At the power tubes I get:
pin 1...-24v
pin 3...399v
pin 4...406v

v1
pin 1...159v
pin 3...1.25v
pin 6...165v
pin 8...1.32

v2
pin 1...159v
pin 3...1.4v
pin 6...160v
pin 8...1.6v

v3
pin 1...406
pin 3+8..3.9v
pin 6407v

v4
pin 1...127v
pin 3...1.9v
pin 6...110v
pin 8...1.9v

v5
pin 1...402v
pin 6...351v There is no voltage at cathodes but I'm assuming that's because I can't activate vibrato.

v6
pin 1...214v
pin 2...31v
pin 3 + 8...51v
pin 6...203v.
pin 7...31v

I'm sure that I am missing some critical measurements for you guys to work your magic, but I wanted to
finally get this posted (I'm running late for work) and go from there.

Thanks for all your help, and I can't wait to see the suggestions.

Mike

[Leo_Gnardo]

First, -24V at output tubes pin 1? That bridges thru a 1K5 resistor to pin 5. That's an awfully low bias voltage.

On the bias supply card, there's a resistor between the transformer lead and rectifier. Has its value drifted upward? It often does, if so . . . replace it.

Can you measure bias current? I'd expect it to be sky high at -24V. Depending on output tubes and their condition I'd expect to see -31 to -40V on a Deluxe reverb.

One fairly safe way of measuring bias. With your amp off, read the resistance of each arm of the output transformer primary. Red to brown, red to blue. Usually around 150 ohms. Then with your amp on and warmed up, measure the voltage same way, red to brown, red to blue. Then get out your handy calculator, use Ohm's law, voltage divided by resistance = current. Ideally you'd be around 20 milliamps. Also remember your calculation will deliver an answer in amps; 20 milliamps will read 0.02 on the calculator. Or multiply the calculator's answer x 1000, that's the current in milliamps.

B - ice picky sound, a common complaint for Deluxe Reverbs. There's a brightening capacitor wired in place between the wiper & signal-in pins on the vibrato channel. You could unsolder one lead on that cap & bend it back so you could easily replace it, there's one spot to remove excess sizzle. Another: find the mix node where 3M3 meets 470K with 220K to ground. You'll find a 10 pF cap across the 3M3. Do the same with that, now evaluate your vib channel's tone, I'll bet it's calmed down noticeably.

[Leo_Gnardo]

There is no voltage at cathodes but I'm assuming that's because I can't activate vibrato.

Woops I forgot to say, excellent work Mike, so far so good!

Vibrato needs to have a short at the vibrato control jack to work. You could clip ground to the jack's electrode to cheat it on, and if that works make a cheater plug. Just an RCA plug with the center shorted to shell with a short piece of wire. If there's still no vibrato, apply your cheater wire or plug, look for the blinking light at one end of the vibrato "bug". No blink, could be a bad neon lamp most likely, or tube, speed pot, resistor or failed cap in the oscillator bridge. If it does blink and you have no vibrato, two possible faults: the depth control pot or the photoresistor at the other end of the vibrato "bug." There's a rash of bad photoresistors these days as they get on to 40 50 or more years old. Poking at the photoresistor's leads with a chopstick may revive it for a moment while you press but if it's a bad contact, you'll have to replace it. I use the photoresistors sold by Antique (tubesandmore.com) intended to repair Morley pedals.

[mozz]

"read the resistance of each arm of the output transformer secondary"

That should be primary windings?

I think you may have some of those 10k power resistors slightly out of value.

[Leo_Gnardo]

"read the resistance of each arm of the output transformer secondary" That should be primary windings?

WOOPS! What would I do without ya mozz??? Thanks ! ! ! and now corrected. I was on my first coffee...

Another note for Mike, if the bias voltage still reads way low and bias current measures high, let's have a look at the resistor on the bias pot. You can bump its value up so the bias control will operate over a higher voltage scale & get your bias current dialed in proper like.

[mikep]

Hey guys,
I got back into the BDR this morning. So I did the measurements that Leo recommended. The primary windings of the output transformer. I came up with 220ohm on both sides. The voltages at both pin 3 were 10 + 13. The voltage at pin 1 of the power tubes were -24v. O.K, so I went to the bias pot and turned it up until I read -35 at pin 1 of the power tubes. Voltages at the plates went
down to 4 + 6. So with my handy calculator, I did the current calculations and happily it seems that the current is somewhere around the 20 milliamp spec. I also disconnected the cap at the 3M3 to reduce some sizzle. The amp sounds much better to my ears and I'm pretty pleased with the sound. There will be more testing and tweaking later in the day. I do have some questions as to how bias works that hopefully someone will have the patience to answer.
1. Does the fact that when I measured from the output primary to the plates of the power tubes (4v+6v) mean that the tubes are not matched? I swapped the tubes and the voltages followed the tube so this was my best guess.
2.What is the negative voltage on pin 1 of the power tubes? Is that called raw bias? Also, how do I know what the individual bias is of each tube? Thanks for the help, I'm getting pretty excited about working on some amps. A recap on a 1484 is next on the list!!

[Leo_Gnardo]

I did the current calculations and happily it seems that the current is somewhere around the 20 milliamp spec. I also disconnected the cap at the 3M3 to reduce some sizzle. The amp sounds much better to my ears and I'm pretty pleased with the sound.

Good morning Mike and I see you've made some good progress. "Sounds much better" and a bias point that's sane, sounds good to me!

1. Does the fact that when I measured from the output primary to the plates of the power tubes (4v+6v) mean that the tubes are not matched? I swapped the tubes and the voltages followed the tube so this was my best guess.

You haven't revealed what resistance you measured on the OT primary, but the voltage readings being that unbalanced, and following as you swapped the tubes tells us yes, output tubes are not matched. Small bother that, all it results in is a low hum that could go away if the tubes were well matched. No need to panic. Some day you may invest in new 6V6's, you'll get a matched pair then.

2.What is the negative voltage on pin 1 of the power tubes? Is that called raw bias? Also, how do I know what the individual bias is of each tube?

You're reading bias voltage on pin 1 - or pin 5 - of your output tubes. "Raw", that's what you measure on the bias filter cap.

You measured the bias current of each tube when you measured the voltage across the OT windings and divided that by the OT winding resistance.

Thanks for the help, I'm getting pretty excited about working on some amps. A recap on a 1484 is next on the list!!

Glad to be of help. And "it's about time" on that 1484 - the original caps on all I've seen have this printed on the side "Guaranteed for one year." Half a century later, think it's time for new ones? It's amazing some of the old ones still work at all.