Traynor YBA-2

[Britishampfan]

Hey there wondering if you could help me make out what makes sense on this schematic.

Power supply they spec a 10k one watt resistor and what looks like a 47k one watt my friend says it`s supposed to be 4.7k and put that in there. Along with some resistors to ground that did not work to clear up a noise issue.

Anyway I`m putting a 40-40-40 new can cap back in, ripping out what he insisted on doing cause it did`nt work.

The amp has a problem with 120hz noise unless I pull the PI tube. To me that says power supply issues. Tubes are good and well matched. Thanks!

Schematic

 

[schmidlin]

http://el34world.com/charts/Schematics/files/traynor/traynor_bassmate_yba2.pdf

 

[Britishampfan]

That is a little clearer than my schematic, looks like 47K to me.

Thank you

 

[schmidlin]

You're welcome. Btw, you have YBA-2a in the title, yet post YBA-2. Incidentally the 2a version has a 12 ohmer in that spot. 47k does seem high to me, tho.

 

[Britishampfan]

It does seem high. My amp says it`s a 2a BUT it only has a volume and a tone knob like the YBA 2 and it does date at 66-67.

I just found another post on another BB where the guy said the carbon comp resistor was a 4700 ohm and drops 50 volts. My original one was burnt. I guess I should do some math and see how close it is. It has been running on the 4.7 just fine. Maybe my friend was right?

 

[schmidlin]

You won't be first to find a typo on a schematic. The 47k would drop the screen voltage so low that you will have very little output, and that makes about zero sense. The 4.7k makes great sense. 12 ohms seems low. Do what sounds best.

 

[Britishampfan]

Thanks yeah, going to get it running again, check some voltages, do some math and find that blasted 120hz ripple.

I did see the 2A schematic and guitar mate which has 12k and 56k resistors listed. I noticed the front of my amp says YBA-2 and the back panel tag on the wood says 2a.

I`m going to say it`s a YBA-2 title changed.

 

[Britishampfan]

Well, I got a chance to get that amp off my bench tonight, I wired in the new can cap and reinstalled 4.7K and 10k resistors on the can cap.

Brought it up slowly on my variac and It sounds pretty good, the hum is way improved, I pulled the resistors to ground off the heater circuit, It did not help. I reinstalled the center tap and star grounded everything to one spot.

Hum is not gone but way improved, it might need new output tubes they test good but might be mismatched and wore.

At 120VAC running of the variac the voltage at the 12ax7 was 230 volts DC the plates of the 6V6GT was 367VAC and the voltage at the cap was 405VDC.

Heater voltage was 6.7VAC I looked at the nameplate of the amp it`s 115 volts so I lowered the variac to 115VAC and sure enough my heater voltage was 6.3VAC

The also brought the B+ down and 190VDC at the 12AX7 and around 340VDC at the
Plate. I did`nt get a change to check the bias on my meter yet, I swapped out some input tubes 12at7 for 12ax7 and I got varying degrees of gain and breakup.

I can start fine tuning I am not sure if I like 230VDC at the 12AX7 or 190VDC. If you remember the value of one of the resistors (R 13) on the can cap was unknown.

 

[blueworm]

There are different versions of the Bassmate, but I think A/B is for head/combo version (for all versions).

I have a schematic for the 6V6 Bassmate that has 4.7K for the 1st dropping resistor (but is R19) and 10K for the next one.

 

[Britishampfan]

Well, the above schematic is very close to my amp I`m just going to go with 4.7K on R 13 and 10K on R 14.

The amp sounds really good, running fine, Just needs some new tubes.

 

[Britishampfan]

I checked the bias today @ 120VAC input

348 volts on the plate 47 milliamps 16.2 watts at idle tubes are 1 milliamp apart.


Does this seem a bit hot or normal for a cathode bias amp?

If I variac down to 110 volts I get 320 volts on the plate 42 Ma current and 13.5 watts at idle filament voltage at 6.18

I suppose that in 1967 wall voltage was 110 VAC the amp is rated for 115VAC today my wall voltage is 123VAC Makes everything high.

 

[ECS]

What output tubes are you using?

16 watts is definitely over max dissipation for a 6V6. A 6V6GTA is rated around 14 watts so you should definitely try to bring that down. You could up the cathode resistor to around 300ohms and see where that gets you. A 14 watt tube should be idling around 12-13 watts in a cathode biased amp like this.

 

[Britishampfan]

Thank you for your reply. I am using 6v6GTA

Yes I`m going to have to up the cathode resistor, I don`t want to carry around a variac for this amp.

I found a cathode resistor calculator online last night it looks to me like the plate dissipation on this amp should be 95 to 100% I`m going to test the voltage drop across the resistor and plug in the numbers and see what the cathode resistor online calculators says. 12 to 13 watts should work good.

 

[ECS]

Idling around 90-95% would be good, don't worry if you're off by a bit, the tubes can handle it.

In terms of voltages being high, you're well within safe territory in terms of plate voltage. You didn't list the heater voltage @ 120v on the mains, but as long as it's under 6.9v, you are ok.

 

[pdf64]

When calculating the plate dissipation, the cathode voltage should be subtracted from the plate voltage.

To reduce the line voltage, consider installing a simple bucker transformer http://www.geofex.com/Article_Folders/vintvolt/vintvolt.htm
Pete

 

[Britishampfan]

Idling around 90-95% would be good, don't worry if you're off by a bit, the tubes can handle it.

In terms of voltages being high, you're well within safe territory in terms of plate voltage. You didn't list the heater voltage @ 120v on the mains, but as long as it's under 6.9v, you are ok.

The filament voltage is a little high running at 120 volts nominal like 6.75- VAC

Ok I will shoot for 90 to 95%.

 

[Britishampfan]

When calculating the plate dissipation, the cathode voltage should be subtracted from the plate voltage.

To reduce the line voltage, consider installing a simple bucker transformer http://www.geofex.com/Article_Folders/vintvolt/vintvolt.htm
Pete

Yeah a little buck boost transformer would work, pretty smart.

 

[Britishampfan]

Well, I did the math and I need a 360 Ohm 10 watt resistor.

I have 25 volts across the resistor. 360 ohm will get me to 11.8 watts plate dissipation running at 98% of max dissipation.

I think that should take care of it.