So, I just finished Guitar amp 3 of 4 today which was a fairly fun build, (Copying from a built amp is so much faster)
Fired it up, Seemed like instant success until I dimmed the lights in the room.
When I cord or puck a string, The orange glow from the tube changes (Gets Brighter). Not to be confused with the filament which stays constant, The tube clearly gets brighter. I also notice a bit of blue glow internally near the base of both output tubes. This also changes with any note or cord.
I had not either noticed this in neither of the other amps or it does not happen.
Its not red plateing but, I have not run it for a prolonged period to be certain.
Now, I have adjustable fixed bias and it has a fair amount of range at the pot, My question is more or less negative voltage?
I just tried to get a voltage reading at pin 3 of the output tube at the socket, The meter was set at 600V DC range and something inside the meter blew ( literally flashed or arced inside).
I have used this meter without issues prior to this incident. Needless to say, it no longer works.
I tried a pair of EL34's but it appears they may even be worse, I thought I even saw the tube arc near the top by this mica. The amp " Sounds Great, but clearly its not correct.
Most of the voltages on the sketch are old references, I used this bias arrangement.
basic output schematic
On my way out for another meter :whazzat:
sounds like the screen dissipation is going high. Thats the only grid that will typically glow on signal peaks. The blue glow thing is totally normal and not of concern. Inserting a larger screen stopper resistor and/or changing the screen supply may be necessary. It may last a while like it is, but you never know... if its glowing its getting too hot.
Darn, no place open here on a sunday night to get a new meter.
I suspect somethings possibly mis-wired or more likely a componet off spec. I guess I will need to wait and get accurate voltages off the key points to see where it is.
My perf-board prototype works perfect and I am fairly certain I followed the design accurately, But, Its hard to re-check wiring under the eyelet board once installed. Removal is virtually disassembly of most of the amp.
I found it Really really odd that the meter blew!
Normally, I test from the ground to pin 3 of the output tube and get normal readings around 400V on the exact settings I used for this amp.
DC 600V scale
On this amp it however took out the meter big time, It litterly flashed inside visible thru the pastic case!! Very Scarey!!
The peak voltage on the plate is the DC voltage (400 volts) PLUS the peak AC signal voltage. The total voltage on signal peaks (under normal conditions) can go to 800 volts. If the amp is mis-loaded (8 ohm load on the 4 ohm tap, or wrong transformer) the voltage can go to several thousand volts. This is bad for output transformers, tubes (arcs) and obviously meters. As stated earlier the pulsating orange glow is likely a screen grid. This can usually be seen through the holes in the plate. The tube will not like this and will eventually fry, often catastrophically.
Check the wiring around the output transformer, tubes, and speaker jack. I have seen 6L6's erupt in fire because I put two 8 ohm speakers in series on the 4 ohm tap. The amps owner really liked the sound it made right before it blew up. We settled on the 8 ohm tap, and 6550's, they would take it.
I have given up on expensive meters, get about 5 of the $5 ones from Harbor Freight. That way you don't get too upset when they blow up. I have only blown one. Ohming out the B+ with the amp on will do it!
First off, the bias voltage for KT77 tubes should not be low enough to blow up a meter.
A typical tube of that type has an adjustable supply that goes no lower than -50V, which is a lot of room. This shouln't cause a problem with a meter rated for 600V.
The DC screen voltage may be too high, see the datasheet for ratings at different screen voltages.
I am 100% certain on the speaker load and correct Tap selected,
It Has 2,4,& 8 ohm selectable output.
I agee the screen grid is the source of the glow, I have seen this before. This one is no where near as bad as a pair of EL84's I melted in my hifi amp.
But it is bad enough that I need to fix it.
The JJ KT77 's have no plate holes to peek thru :( The glow changes are however visible via the mica spacers.
This transformer is rated for a 50W guitar amp so its fairly beefy and heavy. Basically its a multi-tap bassman transformer.
Audio Watts (RMS): 50
Primary Impedance (Ohms): 4,200 CT
Secondary Impedance (Ohms): 2/4/8
Mounting centers: 2" x 3.5" vertical
Weight: 4.7 lbs
I re-checked everything ( Visible ), I have a perfect working unit sitting side by side with this one to compare to.
I'll post side by side pictures, I have re-designed the board, moved a few items, But the basic layout is the same.
Just for piece of mind, I hooked up the original unit, Cranked her up, No Issues. I bought all the matching parts from the same vendor.
The original perf-board version has passed the 3 hour gig test 6 times!
I just dont get it,
But I WILL!!! lol
I have another turret board already populated with all parts, I may replace it entirely.
When I touched the meter probe to pin 3 the amp produced a rather loud thud at the speaker, This time, I will check all the other voltages before trying that again!!
I am betting that its a bad part, or I missed a connection on the board underside somehow.
My adjustment range is basically -37 to -48V.
It was not that bias supply that blew the meter, It was the plate voltage measurement at Pin3 of the KT77's I have previously measure my other amp at 405V on pin 3, 600V scale on meter.
I hate to use the O word, but maybe something is oscillating. That could cause these symptoms.
I didn't see the PSU on your schem. But I think a bad/incorrect (shorted?) inductor could send your B+ through the roof.
Just a thought...
Did the amp sound right?
EDIT: Yeah George.... hold up on the "O" word - :dead:
Okay, yeah, b+ can achieve some really high voltages especially if you're running a high output level.
Trout's mention of possible oscillation is a wise suggestion in my opinion.
I had an amplifier with russian 6146 radio tubes oscillate and actually cause sparking at the screen grid.
With the addition of suitable screen bypass caps right on the sockets, the problem was gone.
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