Before rebuilding my second Fairchild 260, I decided to check some of the voltages on my operational 260. I was surprised at some of the numbers. On pin 1 of the 6AB4 the Fairchild schematic says I should have 65v +/- 15%. I have 75v which is .25v higher than +15%.
On the 12AU7 I'm supposed to have 7.4v on pin 3 but have 8.67 which is .16v higher than +15%. I decided to stay with the 12AU7 even though the distortion levels on this tube are supposed to be higher than the 12BH7a or 6CG7 which pull twice as much heater current. When I tried out a 12BH7, it caused a mechanical hum in the transformer and caused it to get really hot after being turned on for a couple of hours! I'm not sure if it was due to the increased draw, but I decided to stick with what worked for Fairchild.
One thing that freaks me out a little bit is the voltage I see on the EL34s at turn on. As voltage starts to register on pin 3, it spikes to over 560v before settling back down to 513v. 513v is barely within the +15% upper range 517.5v. I have the bias correctly set to .55v at pin 8 of both tubes.
Oh, and I'm getting 523v on pin 8 of the 5V4Gs. If I'm reading it correctly, the Fairchild schematic says it should be 475v. At +/-15% this is within tolerance. However, the multi section caps C6 & C7 are only rated 500v! I'm wondering if I need to worry.
Does anyone think that I should make any changes to bring down the voltages that are above the 15% +/- tolerances? It sounds great as is, but if I need to change some resistor values to improve longevity I might consider doing so.
On the 12AU7 I'm supposed to have 7.4v on pin 3 but have 8.67 which is .16v higher than +15%. I decided to stay with the 12AU7 even though the distortion levels on this tube are supposed to be higher than the 12BH7a or 6CG7 which pull twice as much heater current. When I tried out a 12BH7, it caused a mechanical hum in the transformer and caused it to get really hot after being turned on for a couple of hours! I'm not sure if it was due to the increased draw, but I decided to stick with what worked for Fairchild.
One thing that freaks me out a little bit is the voltage I see on the EL34s at turn on. As voltage starts to register on pin 3, it spikes to over 560v before settling back down to 513v. 513v is barely within the +15% upper range 517.5v. I have the bias correctly set to .55v at pin 8 of both tubes.
Oh, and I'm getting 523v on pin 8 of the 5V4Gs. If I'm reading it correctly, the Fairchild schematic says it should be 475v. At +/-15% this is within tolerance. However, the multi section caps C6 & C7 are only rated 500v! I'm wondering if I need to worry.
Does anyone think that I should make any changes to bring down the voltages that are above the 15% +/- tolerances? It sounds great as is, but if I need to change some resistor values to improve longevity I might consider doing so.
1 question as a starter..is your meters okay / not runs on flat battery?
for the caps, is it getting hot?.if yes, you should stop now.it's dangerous since it might bulging and explode.
for the caps, is it getting hot?.if yes, you should stop now.it's dangerous since it might bulging and explode.
What was main AC net voltage before 30 years ago?
Maybe the el. caps are dry and You can change them.
Is there humm in speakers?
Maybe the el. caps are dry and You can change them.
Is there humm in speakers?
The measurements were taken with a Fluke DVM on a 260 that was rebuilt 3-4 years ago with all new tubes, some of them nos, tube sockets, caps and resistors. The multi-section 'lytics are JJ 500V.
This amp is dead quiet, even at full gain and when you put your ear next to the speaker. It was rated at 50W but puts out 60W. It was designed for use with 117V that was common in the 1950s. Today, my wall plugs read 120-121V.
Check the filament voltage and if it is more than 6,3 reduce your mains voltage with a small external transformer.
WBS, 0.55 v on the cathodes means 25.7 ma in each tube /if cathodes are connected together/, for PP in this class it's maybe OK. All another modes are little bit higher....You can remain them, but only change el. caps in PS to new, but with 600-650 v.
Take a look at the schematic and see whether or not they indicate the ohms per volt (sensitivity of an analog volt meter) of the meter used to make the measurements. (20K per V would have been good for the time) Modern digital meters generally have higher input impedances than old analog meters and load down the circuit less.. Your voltage readings are probably reasonable give this..
Note that line voltage may have some influence on this, but presumably you are using modern power transformers wound for your mains voltage so only the AC line voltage tolerance would be relevant here. (Or the originals with an auto-transformer?)
Voltmeter - Wikipedia, the free encyclopedia
Note that line voltage may have some influence on this, but presumably you are using modern power transformers wound for your mains voltage so only the AC line voltage tolerance would be relevant here. (Or the originals with an auto-transformer?)
Voltmeter - Wikipedia, the free encyclopedia
Thanks for the replies!
Filament voltage is 6.8v which is within the +/- 15% tolerance given by Fairchild.
A bias of .55v on pin 8 is what the Fairchild instructions for conversion to the 260R (EL34 model) call for. I'll look to see what size 600v electrolytics would be. I'm not sure I have any extra room. When I have a chance, I'll take a pic and post it so you can see.
I haven't found any reference to meter requirements in the Fairchild info, just Sam's which specifies a vacuum tube meter measuring 1000 ohms per V and capable of achieving .5V accuracy.
The transformers are original running on my 120-121V house voltage. When I picked it up from the tech after the rebuild, it out 60 watts instead of Fairchild's published specs of 50 watts!
Filament voltage is 6.8v which is within the +/- 15% tolerance given by Fairchild.
A bias of .55v on pin 8 is what the Fairchild instructions for conversion to the 260R (EL34 model) call for. I'll look to see what size 600v electrolytics would be. I'm not sure I have any extra room. When I have a chance, I'll take a pic and post it so you can see.
I haven't found any reference to meter requirements in the Fairchild info, just Sam's which specifies a vacuum tube meter measuring 1000 ohms per V and capable of achieving .5V accuracy.
The transformers are original running on my 120-121V house voltage. When I picked it up from the tech after the rebuild, it out 60 watts instead of Fairchild's published specs of 50 watts!
- Status
- Not open for further replies.