[ook ook]

Hi all, just started working on this '65 bass master. There were some broken solder joints in the amp when I got it, so my hope was that these would be the only issues. I fixed up the solder joints that were clearly broken and ran the amp through a current limiter & variac. It did not produce any sound at all. This was around a week or two ago, but today I did some more troubleshooting.

Still running the amp through the current limiter, I measured some voltages. The voltages are low. Insanely low. With all tubes in, I get a B+ of ~300V. My 6.3V filament voltage is only around 5VAC and my 5V filament is at ~3.9VAC.

With only the rectifier tube in, I have 6.4VAC and 5VAC with a B+ around 475V. These values stay in that ballpark if only the 12ax7s are in. The voltages drop like crazy once the power tubes go in. I have three sketchy 7027a tubes that I have tried in there. I plan to do a conversion to el34s, but I haven't gotten around to doing so yet.

When the amp is powered on without power tubes in, the amp itself emits a 120-cycle hum. When the power tubes are in, there is absolutely no sound.

Sometimes when the amp is switched on, there is some popping which can be heard from the speaker immediately, but it goes dead silent afterwards.

One time when it was switched on, I heard a low frequency signal come through the speaker which had a tremolo effect to it after it had been on for a few seconds, but I quickly switched it off.

There seems to have been quite a lot of amateur work done in this thing, but the connections all seem fine to me. I haven't checked every single solder joint, but the ones I have checked seem fine (not pretty, but functional).

I tried to avoid dropping a wall of text and just get straight to the point, but I'm not very good at that. Anybody have any suspicions as to what I should look into? I'm hoping and praying it's not a dying transformer. Could it be filter caps or something like that? They have been replaced with JJs, but I'm not exactly sure when. At least 5 years ago, maybe around 10 or even 15.

The amp basically follows the factory schematic exactly. It has the earlier power supply with the tube rectifier as well.

 

[Paul Running]

Still running the amp through the current limiter, I measured some voltages. The voltages are low. Insanely low. With all tubes in, I get a B+ of ~300V. My 6.3V filament voltage is only around 5VAC and my 5V filament is at ~3.9VAC.

With only the rectifier tube in, I have 6.4VAC and 5VAC with a B+ around 475V.

Those symptoms would indicate that the power transformer is overloading. The Hammond equivalent for the YBA-1 is rated 500mA, at full mains voltage for the HT supply.
Verify this observation by load-testing the PS, separately.
Make sure that the PS is healthy first...the first test for any electronic unit.
Isolate the load to the PS, connect a resistive load to the PS and test.
If no load voltage is 475VDC, with full mains. Full load should not reduce the output to less than 450VDC.
A full-load test for the HT supply requires 1kΩ @ 250w.

 

[ook ook]

A full-load test for the HT supply requires 1kΩ @ 250w.

I need a 1k 250 watt resistor?

 

[nonreverb]

Hi all, just started working on this '65 bass master. There were some broken solder joints in the amp when I got it, so my hope was that these would be the only issues. I fixed up the solder joints that were clearly broken and ran the amp through a current limiter & variac. It did not produce any sound at all. This was around a week or two ago, but today I did some more troubleshooting.

Still running the amp through the current limiter, I measured some voltages. The voltages are low. Insanely low. With all tubes in, I get a B+ of ~300V. My 6.3V filament voltage is only around 5VAC and my 5V filament is at ~3.9VAC.

With only the rectifier tube in, I have 6.4VAC and 5VAC with a B+ around 475V. These values stay in that ballpark if only the 12ax7s are in. The voltages drop like crazy once the power tubes go in. I have three sketchy 7027a tubes that I have tried in there. I plan to do a conversion to el34s, but I haven't gotten around to doing so yet.

When the amp is powered on without power tubes in, the amp itself emits a 120-cycle hum. When the power tubes are in, there is absolutely no sound.

Sometimes when the amp is switched on, there is some popping which can be heard from the speaker immediately, but it goes dead silent afterwards.

One time when it was switched on, I heard a low frequency signal come through the speaker which had a tremolo effect to it after it had been on for a few seconds, but I quickly switched it off.

There seems to have been quite a lot of amateur work done in this thing, but the connections all seem fine to me. I haven't checked every single solder joint, but the ones I have checked seem fine (not pretty, but functional).

I tried to avoid dropping a wall of text and just get straight to the point, but I'm not very good at that. Anybody have any suspicions as to what I should look into? I'm hoping and praying it's not a dying transformer. Could it be filter caps or something like that? They have been replaced with JJs, but I'm not exactly sure when. At least 5 years ago, maybe around 10 or even 15.

The amp basically follows the factory schematic exactly. It has the earlier power supply with the tube rectifier as well.

How much current limiting are you applying to the source voltage? If it's excessively limited, loading the amp could cause some of what you're describing.
As for the hum, something is singing. I find old Traynor power transformers sometimes have loose windings causing them to do what you're describing.

 

[tomee2]

Do you know that the output tubes are not running at a very high DC current because of leaky coupling capacitors?

 

[ook ook]

How much current limiting are you applying to the source voltage? If it's excessively limited, loading the amp could cause some of what you're describing.
As for the hum, something is singing. I find old Traynor power transformers sometimes have loose windings causing them to do what you're describing.

Running the amp through a light bulb limiter with a 100 watt bulb. The bulb does not glow near its full brightness

Do you know that the output tubes are not running at a very high DC current because of leaky coupling capacitors?

I'll have to check the caps

 

[nonreverb]

Running the amp through a light bulb limiter with a 100 watt bulb. The bulb does not glow near its full brightness


I'll have to check the caps

Yup, that should be plenty. Although the fact that the B+ is plunging when the power tubes are installed is troubling.
What's the static resistance of the primary windings? Also, is there any resistance between ground and any windings? I've had extremely rare cases where once the transormer is passing current, it conducts to bad places due to bad insulation on one of the windings....long shot I know but it can happen. Might as well eliminate anything O/P transformer related....

 

[ook ook]

Yup, that should be plenty. Although the fact that the B+ is plunging when the power tubes are installed is troubling.
What's the static resistance of the primary windings? Also, is there any resistance between ground and any windings? I've had extremely rare cases where once the transormer is passing current, it conducts to bad places due to bad insulation on one of the windings....long shot I know but it can happen. Might as well eliminate anything O/P transformer related....

From the plates to the center tap of the output transformer, one side measures ~35 ohms and the other measures ~40 ohms. I think I had already checked the resistance to ground and found nothing alarming. I haven't done so while the transformer is passing current

 

[ook ook]

Oh yeah, and I also noticed that the low input on the normal channel shows a DC resistance of 1.2 Mohms from tip to ground. The high input is around 1M for both channels, and the low input on the treble channel is 140k. Not sure what the deal is here. It's hard to see the wiring properly, but it looks fine

 

[Latole]

If power tubes are running ( are bias ) too hot ; voltageS will drop.
What are the cathod current on theses tubes ?

____
"When the amp is powered on without power tubes in, the amp itself emits a 120-cycle hum. When the power tubes are in, there is absolutely no sound."
-ook ook

Many issues on this amp ; can you post good pictures ?

 

[Paul Running]

I need a 1k 250 watt resistor?

475VDC / 500mA = 950Ω, 475VDC × 500mA = 238W.

 

[ook ook]

If power tubes are running ( are bias ) too hot ; voltageS will drop.
What are the cathod current on theses tubes ?

____
"When the amp is powered on without power tubes in, the amp itself emits a 120-cycle hum. When the power tubes are in, there is absolutely no sound."
-ook ook

Many issues on this amp ; can you post good pictures ?


View attachment 460109

I can take some pictures next time I pull the amp out.

475VDC / 500mA = 950Ω, 475VDC × 500mA = 238W.

Makes sense to me

 

[ook ook]

475VDC / 500mA = 950Ω, 475VDC × 500mA = 238W.

I don't have anything comparable on hand. Any suggestions as to where I could get something like this for a reasonable price?

If power tubes are running ( are bias ) too hot ; voltageS will drop.
What are the cathod current on theses tubes ?

____
"When the amp is powered on without power tubes in, the amp itself emits a 120-cycle hum. When the power tubes are in, there is absolutely no sound."
-ook ook

Many issues on this amp ; can you post good pictures ?


View attachment 460109

I checked the current with a bias probe hooked up to my multimeter and I got a measly 14 mA for both tubes. With plate voltages at 300V, that's like 4 watts dissipation. I don't need to tell you guys that's low.

I pulled the rectifier and took some AC voltage measurements. With full line voltage and no tubes in, I'm getting these values on my PT secondary taps:
340-0-340
6.45V
5.19V

With the rectifier and preamp tubes, it was more like:
330-0-330
6.3V
5.0V

I also took some pictures. This amp has certainly had some pretty questionable work done on it in the past. Hopefully I can rectify that. If we need close-up shots of anything in particular, I can do that.
The pictures are gonna be in this spoiler below since they'll be pretty large

Spoiler: Pictures

 

[Paul Running]

I don't have anything comparable on hand. Any suggestions as to where I could get something like this for a reasonable price?

I scrounge most of my parts from discarded gear or I buy at auctions.
A power decade resistance box or power rheostat can be handy for load testing. I made a load-tester from a combination of power resistors and rheostats.
Most of the load resistors that I use for load-testing are made from series or paralleling power resistors. Nothing fancy but I just use what I have: The top one is just two rheostats in series, adjustable from 0 - 4K22Ω. The bottom one is 2 - 10KΩ resistors in parallel.
The rheostats are good because you can decrease the resistance progressively.
There are several methods to provide a load.
If you are willing to spend the time and money, you could design and build a superb load box.
An active load box would be ideal.

 

[nonreverb]

I don't have anything comparable on hand. Any suggestions as to where I could get something like this for a reasonable price?


I checked the current with a bias probe hooked up to my multimeter and I got a measly 14 mA for both tubes. With plate voltages at 300V, that's like 4 watts dissipation. I don't need to tell you guys that's low.

I pulled the rectifier and took some AC voltage measurements. With full line voltage and no tubes in, I'm getting these values on my PT secondary taps:
340-0-340
6.45V
5.19V

With the rectifier and preamp tubes, it was more like:
330-0-330
6.3V
5.0V

I also took some pictures. This amp has certainly had some pretty questionable work done on it in the past. Hopefully I can rectify that. If we need close-up shots of anything in particular, I can do that.
The pictures are gonna be in this spoiler below since they'll be pretty large

Spoiler: Pictures

View attachment 460950

View attachment 460952

View attachment 460953

View attachment 460948

View attachment 460949

View attachment 460951

Wow....what a mess. You might want to check that all the cathodes of the preamp tubes are actually grounded properly. Actually, I'd be inspecting all the ground points for continuity for starters...

 

[diyfabtone]

If you have a blown speaker around you can cut the cone out and use it as a load for testing as well. 8ohm/50+W should be sufficient. Bonus is its a reactive load, not purely resistive.

 

[ook ook]

Wow....what a mess. You might want to check that all the cathodes of the preamp tubes are actually grounded properly. Actually, I'd be inspecting all the ground points for continuity for starters...

Good call. Somebody's done a number on this thing.

If you have a blown speaker around you can cut the cone out and use it as a load for testing as well. 8ohm/50+W should be sufficient. Bonus is its a reactive load, not purely resistive.

(Un)fortunately, I don't have any blown speakers

I scrounge most of my parts from discarded gear or I buy at auctions.
A power decade resistance box or power rheostat can be handy for load testing. I made a load-tester from a combination of power resistors and rheostats.
Most of the load resistors that I use for load-testing are made from series or paralleling power resistors. Nothing fancy but I just use what I have: The top one is just two rheostats in series, adjustable from 0 - 4K22Ω. The bottom one is 2 - 10KΩ resistors in parallel.
The rheostats are good because you can decrease the resistance progressively.
There are several methods to provide a load.
If you are willing to spend the time and money, you could design and build a superb load box.
An active load box would be ideal.

View attachment 460967

I'll look into it, but I'm not looking to spend a bunch of money on something like that right now

 

[Paul Running]

People will have their own methods to test electronic gear. In my past experience, I learned to ensure that the power supply is fully serviceable, before moving on to anything else.
Once there is confidence in the power supply's performance then, I move on to the output stage and work in reverse to the input stage.

I checked the current with a bias probe hooked up to my multimeter and I got a measly 14 mA for both tubes. With plate voltages at 300V, that's like 4 watts dissipation.

What is the static voltage at the grids of the 7027As?

 

[nonreverb]

Just on a whim....did you happen try another tube rectifier in it? There's the possibility it's completely pooched.

 

[ook ook]

People will have their own methods to test electronic gear. In my past experience, I learned to ensure that the power supply is fully serviceable, before moving on to anything else.
Once there is confidence in the power supply's performance then, I move on to the output stage and work in reverse to the input stage.

What is the static voltage at the grids of the 7027As?

When the tubes are pulled, the grids are at -37V. Haven't checked with the tubes installed, but I will next time I'm poking around in there

Just on a whim....did you happen try another tube rectifier in it? There's the possibility it's completely pooched.

I haven't tried any other rectifiers. This one's brand new, although I know that doesn't necessarily mean it's good. I don't have any replacements on hand