High B+ Voltage in Build

jlatrace · 11-03-2009, 04:29 AM

I need help troubleshooting a build. It's a 6v6 power section, a When I fire it up I am getting high AC and DC voltages. Also my bias voltage is very low. I have swapped the PT tranny lines, retraced phase inverter wiring and double checked the bias filter to make sure positive was running to ground. I swapped the PT and the problem remained. I can't find an issue other than my voltages are way too high.

I've used a harvested OT and am about to pull it and try an OT I know work.

Before I do this does anyone hav any other suggestions?

tubeswell · 11-03-2009, 05:10 AM

Quote:

Originally Posted by jlatrace

I need help troubleshooting a build. It's a 6v6 power section, a When I fire it up I am getting high AC and DC voltages.

What AC and DC voltages are you getting specifically? And where are you measuring them?

Quote:

Originally Posted by jlatrace

Also my bias voltage is very low. I have swapped the PT tranny lines, retraced phase inverter wiring and double checked the bias filter to make sure positive was running to ground.

What do you need to check the phase inverter specifically for? Are you concerned with the phase inverter voltages? What sort of phase inverter is it? What are the plate and cathode DC voltages at idle of the PI? What is the DC bias voltage for the output tubes? What is the tube current? Are the plates glowing red? What are the DC idle voltages at the cathodes, screens and plate of the 6V6s (measured from the relevant socket pins to ground)? What is the accurate measured resistance value of the cathode resistor? Are the 6V6 cathodes sharing one resistor? etc etc

Quote:

Originally Posted by jlatrace

I swapped the PT and the problem remained. I can't find an issue other than my voltages are way too high.

I've used a harvested OT and am about to pull it and try an OT I know work.

Before I do this does anyone hav any other suggestions?

More information is needed about what you are trying to build. Can you please post a schematic? What are the AC voltages on your PT secondaries? What is your B+ DC voltage?

Prairie Dawg · 11-03-2009, 05:12 AM

Quote:

Originally Posted by jlatrace

I need help troubleshooting a build. It's a 6v6 power section, a When I fire it up I am getting high AC and DC voltages. Also my bias voltage is very low. I have swapped the PT tranny lines, retraced phase inverter wiring and double checked the bias filter to make sure positive was running to ground. I swapped the PT and the problem remained. I can't find an issue other than my voltages are way too high.

I've used a harvested OT and am about to pull it and try an OT I know work.

Before I do this does anyone hav any other suggestions?

How high is "high"? Are you checking it with the tubes installed and hooked up to a load? Reason I ask is I have an amp I'm working on for a guy, without tubes it was reading 525v on the plates and installing the tubes dropped it right down to about 425.

Chuck H · 11-03-2009, 06:28 AM

Sounds like a rectifier problem to me.

The post title reads "High B+ Voltage in Build". Is this a new build then? Did it ever work correctly?

Chuck

loudthud · 11-03-2009, 11:00 PM

What is the heater voltage coming out of the transformer?

jlatrace · 11-03-2009, 11:53 PM

Thank for all the replies. I am attaching a schematic of my build that I put together today. Tonight I will take some more voltage readings. I noted the voltage readings taken last night in red. I have not tubed the amp bcause of the B+ voltage issue.

tubeswell · 11-04-2009, 12:33 AM

Right - you have a PT with an Ht VAC of 354-0-354 and you are using SS rectification and you are getting 500V B+, so your 450V filter caps and your 6V6s won't last long at that rate. having said that when you put the tubes in, the voltages should drop by 20-30V I guess, but still probably too high for 6V6s (unless they are JJ6V6S - but that is still pushing it).

Neverthelss you can put those 32uF filter caps in series, with a 220k2W resistor in parallel with each cap to equalize the voltage over both caps - this will make the reservoir caps safe, but you will only end up with 16uF reservoir, but that shouldn't matter. (You might have to do the same sort of thing with all the filter caps if the DC voltage on them is above the rated maximum for the caps.

You can put 6L6s in there but you will want to adjust the bias accordingly and you will want the OT load resistance to be about half to 2/3 of what the 6V6 OT load resistance is - one way of doing that is to can halve the speaker load on the existing OT.

Another way of getting the B+ down is a string of reverse-biased zener diodes in series with the centre tap of the 354-0-354 winding to ground. To begin with try say half a dozen 5W 15V zeners in series. Have the banded ends (cathodes) pointing towards the chassis ground. That should drop the B+ by 90V or so, which will make is safe for 6V6s and safe for your filter caps

jlatrace · 11-04-2009, 01:09 AM

Thanks. Interestingly I pulled the ot from a tweed champ and its B+ at pin 3 was 388. I think I have a bias issue that is staring me ib the face but I cabt see it.

Enzo · 11-04-2009, 01:49 AM

If there are no tubes in it, any bias situation won;t matter. The only voltage readings that matter are taken with tubes in the amp.

jlatrace · 11-04-2009, 05:06 AM

I had a 1 meg resistor range resistor in my bias section by mistake. The problem is found and solved.

11-03-2009, 04:29 AM	#1
jlatrace Member Join Date: Jan 2007 Location: Birmingham Alabama Posts: 56	High B+ Voltage in Build - Can't Find Issue I need help troubleshooting a build. It's a 6v6 power section, a When I fire it up I am getting high AC and DC voltages. Also my bias voltage is very low. I have swapped the PT tranny lines, retraced phase inverter wiring and double checked the bias filter to make sure positive was running to ground. I swapped the PT and the problem remained. I can't find an issue other than my voltages are way too high. I've used a harvested OT and am about to pull it and try an OT I know work. Before I do this does anyone hav any other suggestions?

11-03-2009, 11:00 PM	#5
loudthud Supporting Member Join Date: May 2006 Location: Texas USA Posts: 796	What is the heater voltage coming out of the transformer? __________________ WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

11-04-2009, 12:33 AM	#7
tubeswell Supporting Member Join Date: Feb 2007 Location: Wellington NZ Posts: 2,351	Right - you have a PT with an Ht VAC of 354-0-354 and you are using SS rectification and you are getting 500V B+, so your 450V filter caps and your 6V6s won't last long at that rate. having said that when you put the tubes in, the voltages should drop by 20-30V I guess, but still probably too high for 6V6s (unless they are JJ6V6S - but that is still pushing it). Neverthelss you can put those 32uF filter caps in series, with a 220k2W resistor in parallel with each cap to equalize the voltage over both caps - this will make the reservoir caps safe, but you will only end up with 16uF reservoir, but that shouldn't matter. (You might have to do the same sort of thing with all the filter caps if the DC voltage on them is above the rated maximum for the caps. You can put 6L6s in there but you will want to adjust the bias accordingly and you will want the OT load resistance to be about half to 2/3 of what the 6V6 OT load resistance is - one way of doing that is to can halve the speaker load on the existing OT. Another way of getting the B+ down is a string of reverse-biased zener diodes in series with the centre tap of the 354-0-354 winding to ground. To begin with try say half a dozen 5W 15V zeners in series. Have the banded ends (cathodes) pointing towards the chassis ground. That should drop the B+ by 90V or so, which will make is safe for 6V6s and safe for your filter caps __________________ Building a better world (one tube amp at a time)

11-04-2009, 01:49 AM	#9
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,271	If there are no tubes in it, any bias situation won;t matter. The only voltage readings that matter are taken with tubes in the amp. __________________ Education is what you're left with after you have forgotten what you have learned.

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11-03-2009, 06:28 AM	#4
Chuck H Old Timer Join Date: May 2007 Location: pacific north west Posts: 1,226	Sounds like a rectifier problem to me. The post title reads "High B+ Voltage in Build". Is this a new build then? Did it ever work correctly? Chuck

11-04-2009, 01:09 AM	#8
jlatrace Member Join Date: Jan 2007 Location: Birmingham Alabama Posts: 56	Thanks. Interestingly I pulled the ot from a tweed champ and its B+ at pin 3 was 388. I think I have a bias issue that is staring me ib the face but I cabt see it.

11-04-2009, 05:06 AM	#10
jlatrace Member Join Date: Jan 2007 Location: Birmingham Alabama Posts: 56	I had a 1 meg resistor range resistor in my bias section by mistake. The problem is found and solved.

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