'65 DRRI Hand Wired Conversion Issues

[BSG1]

I recently completed my first conversion of a '65 DRRI from pcb (the main board had been butchered by someone's modding efforts) to hand wired using eyelet boards and small parts kit from MOJOTONE. In a shameless plug for the tech guys at MOJO, they have been very helpful from the beginning, but this one has them scratching their heads too.

The amp works fine; chassis on the bench with all new TungSol tubes, 8 ohm test speaker hooked up, footswitch connected and reverb tank connected. The issue(s) which are totally driving me nuts are:
1) The volume pot on the normal channel pops when I turn it off the 1 but works flawlessly from 2-10.
2) The volume pot on the Vibrato channel pops when I turn it to the 10 but it works flawlessly all the way from 1-9.
It's almost as if the pop is like you might hear from the on/off power or standby switches, but these are standard new CTS pots not switches.

I have checked grounds:
1) Separate ground buss wires for each channel.
2) All pots are securely tightened to the chassis with star washers on the inside of the chassis.
3) Ground buss wires are soldered to the backs of the new CTS pots are secure.
4) Ground buss wires are securely soldered to the shields of the input jacks.
5) Grounds from the pots to the buss wire checked and secured.

I'd really like got this project 100% and back into the cabinet, but I wouldn't be able to live with popping volume pots.

Ideas warmly welcomed, please.

 

[Lowerleftcoast]

It may just be the pot. I have noticed some pots do not exhibit a smooth transition from the resistance material to the metal *end* of the resistive track. I have even seen this phenomenon when checking the sweep with a multimeter. (I have also heard it with volume pots in guitars. Not a pop but an uneven volume sweep.)

 

[Ten Over]

I have a bunch of them that don't do anything until I get towards 2 and then they begin their sweep. I had one of them that would go full volume as soon as I turned it from 1 and then it would begin its sweep. I had it for the Master Volume in a 100W Marshall. I'm sure the neighbors didn't appreciate my repeated testing.

 

[Lowerleftcoast]

As a test...
try clipping in a high resistance, say 1M or more, from the normal channel vol pot middle lug to the ground side of that pot.

try clipping in a high resistance, say 1M or more, from the vibrato channel vol pot middle lug to the signal side of that pot.

As a concern for a possible capacitor leak, check for DC voltage on the signal side.

 

[BSG1]

It may just be the pot. I have noticed some pots do not exhibit a smooth transition from the resistance material to the metal *end* of the resistive track. I have even seen this phenomenon when checking the sweep with a multimeter. (I have also heard it with volume pots in guitars. Not a pop but an uneven volume sweep.)

I agree. I recently built a 5E3 and had a bad CTS pot fresh off the shelf. It was supposed to be a 1M but only went to 512k and the sweep was erratic...

I guess, it's possible that I now have 2 more bad CTS pots. Is there somebody selling cheap fakes? I wonder?

 

[BSG1]

As a test...
try clipping in a high resistance, say 1M or more, from the normal channel vol pot middle lug to the ground side of that pot.

try clipping in a high resistance, say 1M or more, from the vibrato channel vol pot middle lug to the signal side of that pot.

As a concern for a possible capacitor leak, check for DC voltage on the signal side.

Thank you, I will give these a try tonight.

 

[BSG1]

I have a bunch of them that don't do anything until I get towards 2 and then they begin their sweep. I had one of them that would go full volume as soon as I turned it from 1 and then it would begin its sweep. I had it for the Master Volume in a 100W Marshall. I'm sure the neighbors didn't appreciate my repeated testing.

If you were my neighbor and you're playing good old rock n roll you could test away all you like!

 

[King Fan]

I agree. I recently built a 5E3 and had a bad CTS pot fresh off the shelf. It was supposed to be a 1M but only went to 512k and the sweep was erratic...

I guess, it's possible that I now have 2 more bad CTS pots. Is there somebody selling cheap fakes? I wonder?

That 5E3 pot sounds really bad. But for the pop or other discontinuity at the ends of the track, I think about it differently. I'm not sure those would need to be cheap fakes. It seems like it's pretty common in vintage CTS pots, too -- as noted by @Lowerleftcoast, the ends of the track have always been a tricky area to make mechanically smooth and electrically continuous. The 0-60 in 1 second (of arc!!) mentioned by @Ten Over is *very* common.

Let us know what you find with LLC's good idea of testing with resistors...

 

[BSG1]

That 5E3 pot sounds really bad. But for the pop or other discontinuity at the ends of the track, I think about it differently. I'm not sure those would need to be cheap fakes. It seems like it's pretty common in vintage CTS pots, too -- as noted by @Lowerleftcoast, the ends of the track have always been a tricky area to make mechanically smooth and electrically continuous. The 0-60 in 1 second (of arc) mentioned by @Ten Over is *very* common.

Let us know what you find with LLC's good idea of testing with resistors...

Well, using the suggested resistance tests I determined that the pots are fine, thank goodness. I wasn't looking forward to pulling them out. So, I took yet another look at my solder joint quality and decided to reflow a few that looked "iffy" from the topside especially those between the large orange drop caps.

Then I did a thorough shake out to get any small bits that might be hiding out. I set it up and started the recommended first fire up procedure:
1) No tubes, power on no issues got the 6V fil heater to each socket.
2) Power off, install GZ34 and power on. No issues and 430V at the B+ (whatever B+ means) which is what my RobRob annotated layout shows.
3) Power off, install 6V6's and power on. WOW! I was getting 290V before and only 8 watts with the bias pot pegged; now I am at 405V and 10 watts right in the middle of the bias pots swing. I don't know what I didn't have soldered correctly, but this is one heck of an improvement.
4) Power off, install pre-amp tubes and power on. Voltages checked but I didn't have the reference layout and didn't record them.

I didn't want to go to bed not knowing how it sounded, but I was too tired to be disappointed and too tired to be working on high voltage stuff...

Tonight we'll get at it again.
Thanks to all for your help so far.

 

[Lowerleftcoast]

whatever B+ means

B stands for battery. In days of old, the power to the tube was supplied by a battery. So B+ is the plus side of the battery. Our modern amplifier rectifies AC to DC so DC is still used to power the tubes but it is not a battery. The terminology is still used. B+ is the full DC voltage supplied. B+1, B+2, and so on are lower DC voltages denoting the differing voltage nodes delivering usually progressively lower DC voltages.

So B+ is the voltage right after the rectifier. Usually it will be found at the positive end of the first filter capacitor, which is known as the reservoir capacitor.

 

[Ten Over]

(whatever B+ means)

Triodes used to be powered by three batteries. The "A" battery powered the filaments (aka heaters). The "B" battery supplied the positive voltage for the plates. The "C" battery supplied the negative voltage for the grid bias. This is why the graphs in the data sheets label the grid voltage "Ec" and the plate voltage "Eb". The heater voltage is typically referred to as "Ef" because it is usually AC instead of the original DC from a battery.

The "B" in "B+" has been carried over from the "B" battery. Each succeeding generation has seen fit to bastardize the notational system for their own purposes so that now "B+" doesn't have a rock solid definition and people use it in various different ways. It usually seems to have something to do with the high voltage DC in amplifiers, however.

 

[Dacious]

Do you have a proper grounding plate or scheme? The originals have a brass plate under the pots soldered by braid to the chassis. Do you have a ground on the pot backs?

Could be as simple as a ground issue on the pots.

 

[King Fan]

Do you have a proper grounding plate or scheme? The originals have a brass plate under the pots soldered by braid to the chassis. Do you have a ground on the pot backs?

Could be as simple as a ground issue on the pots.

Heh, I'm a little OCD about grounds (some folks are even more). FWIW, I see he says,
"I have checked grounds:
1) Separate ground buss wires for each channel.
2) All pots are securely tightened to the chassis with star washers on the inside of the chassis.
3) Ground buss wires are soldered to the backs of the new CTS pots are secure.
4) Ground buss wires are securely soldered to the shields of the input jacks.
5) Grounds from the pots to the buss wire checked and secured."

Complicated. So he has plenty of grounds -- of course, that's no guarantee it's not a ground problem.

 

[BSG1]

Do you have a proper grounding plate or scheme? The originals have a brass plate under the pots soldered by braid to the chassis. Do you have a ground on the pot backs?

Could be as simple as a ground issue on the pots.

Yes, and I checked they are secure. All pots have star washers on the inside and torqued down tight. I soldered buss wires from the jacks to the backs of each pot Normal and Vibrato separate busses. Power ground to the chassis near the tranny.
Thanks.

 

[King Fan]

I didn't want to go to bed not knowing how it sounded, but I was too tired to be disappointed and too tired to be working on high voltage stuff...

Tonight we'll get at it again.
Thanks to all for your help so far.

Good safety move. My biggest amp mistake was made working late into the night -- not a safety thing, thankfully, but amp didn't work right for two weeks after.

If I'm reading right, you think you *may* have fixed the problem? But more stepwise startup awaits? Either way, we're rooting for ya.

 

[BSG1]

Okay, I started over last night! Same start up drill as above.
Caveat! I forgot to mention that I am using the two OEM DRRI transformers and choke I haven't spent the bucks yet for new ones.

With V9 Rectifier only:
467.5V at pin 8
Bias Pot -34.1V
6v at all filaments

With V9 and all 9 preamp tubes (no power tubes) installed pins 1&6:
V1 170V/190V
V2 195.8V/199.1V
V3 439V/439V
V4 197.2V/201V
V5 451V/400V (Houston, we've got a PROBLEM)
V6 200V/200V

I think I cooked V5 with my meters test probe, loud pot and lights out. The rest of the tubes continued to glow.
I went to bed to think about it.

BTW I did retest the two volume pots...they are not popping so reflowing some joints seems to have resolved that issue.

Thanks for your thoughts, as usual.

 

[BSG1]

Good safety move. My biggest amp mistake was made working late into the night -- not a safety thing, thankfully, but amp didn't work right for two weeks after.

If I'm reading right, you think you *may* have fixed the problem? But more stepwise startup awaits? Either way, we're rooting for ya.

Thanks! I appreciate that...note my post this morning. I have an obvious issue, now how to fix it?
You guys are fantastic.

 

[King Fan]

Thanks! I appreciate that...note my post this morning. I have an obvious issue, now how to fix it?
You guys are fantastic.

Bummer. While smart folks think about the voltages, let me ask: are you using a light bulb limiter for startup?

 

[BSG1]

Bummer. While smart folks think about the voltages, let me ask: are you using a light bulb limiter for startup?

Haha...funny you ask. I recently built a 5E3 and on the first fire up, my current limiter glowed faintly. I posted the situation and the responses were like yeah it should glow...I felt kinda stupid.

But, yup I did use the current limiter on the first start up after assembly. My 300W incandescent photo bulb glowed.

 

[Ten Over]

V5 451V/400V (Houston, we've got a PROBLEM)

You undoubtedly have the tremolo off either by the footswitch or by no footswitch connected. With the bias supply voltage on both grids, very little to no current is flowing through the triodes so that there is very little voltage drop across the plate resistors. Those voltages would be expected under these circumstances.