Help With Ampeg Pilot Lamp

[guitom]

What about cathode stripping with the solid state rectifier?

[Stevem]

In my years of repairing and restoring tube amps I have seen output tube failures where 85 percent of the time the amp was paying fine before it was turned off, and the next time it was turned on it instantly blew the the fuse.

This type of failure ensues predominantly from repeated expansion and contraction of the internal parts of the tube.

In my opinion a tube amp need a standby switch to allow a two stage warm up to the tubes .

The first is just with the Heaters being on for a 4 minute period, then the V+ is applied which heats up and expands the internal parts of the tube even more.

After two minutes in standby, then it’s play time.

When turning off the amp the times should be reversed, let the amp sit for two minutes before taking the amp out of play mode and let it sit like that for 4 minutes and after that kill the main power.

This method especially helps when your running the output tubes screen current up close to the max spec.

Go ahead a call me nuts, but there’s no doubt that enough cycles of expansion and contraction in the least and in and of itself will make any audio tube microphonic .

[Phil_S]

What about cathode stripping with the solid state rectifier?

There is nothing to support that as a guitar amplifier problem. I believe it is passed along from broadcast tubes (obsolete for a long time) where it was a real thing. Broadcast tubes carry considerably higher voltages and I think the physical size of them was quite large.

It is hard to argue with Stevem . He might be right; I don't know. Nevertheless, the prevailing thinking is that a standby isn't needed. So, you choose what you like. If you want to wire the standby, you still don't need such a complicated arrangement. Use it to interrupt the B+ supply between the rectifier and the first power supply node. This will allow the heaters to receive voltage at power on so you can allow them to warm up. If there is an existing double pole switch, you can bridge the right and left sides so that you get the benefit of the full current carrying capacity of the switch.

[pompeiisneaks]

What about cathode stripping with the solid state rectifier?

There is nothing to support that as a guitar amplifier problem. I believe it is passed along from broadcast tubes (obsolete for a long time) where it was a real thing. Broadcast tubes carry considerably higher voltages and I think the physical size of them was quite large.

It is hard to argue with Stevem . He might be right; I don't know. Nevertheless, the prevailing thinking is that a standby isn't needed. So, you choose what you like. If you want to wire the standby, you still don't need such a complicated arrangement. Use it to interrupt the B+ supply between the rectifier and the first power supply node. This will allow the heaters to receive voltage at power on so you can allow them to warm up. If there is an existing double pole switch, you can bridge the right and left sides so that you get the benefit of the full current carrying capacity of the switch.

Cathode stripping ONLY happens at over 1200VDC if I recall the data I've read about it. In other words only on tubes never used in guitar amps.

[pompeiisneaks]

In my years of repairing and restoring tube amps I have seen output tube failures where 85 percent of the time the amp was paying fine before it was turned off, and the next time it was turned on it instantly blew the the fuse.

This type of failure ensues predominantly from repeated expansion and contraction of the internal parts of the tube.

In my opinion a tube amp need a standby switch to allow a two stage warm up to the tubes .

The first is just with the Heaters being on for a 4 minute period, then the V+ is applied which heats up and expands the internal parts of the tube even more.

After two minutes in standby, then it’s play time.

When turning off the amp the times should be reversed, let the amp sit for two minutes before taking the amp out of play mode and let it sit like that for 4 minutes and after that kill the main power.

This method especially helps when your running the output tubes screen current up close to the max spec.

Go ahead a call me nuts, but there’s no doubt that enough cycles of expansion and contraction in the least and in and of itself will make any audio tube microphonic .

Honestly, a standby switch would cause this to happen faster. The filaments are warmed up and when standby is switched all the things hit the tubes instantly and hard. Without a standby it is a nice slow gradual warm up of the heaters and a nice slow gradual conductivity in the tube i.e. a nice slow warmup/startup for the anode/cathode/grids etc. So to me that's the opposite of your point here. no?

In any case the failure mode of the tube you're describing is factual, and does happen with repeated 'stress' on any metal, an heating and cooling metals significantly is a stress. I have read a ton on this and keep trying to help people understand the standby switch is a myth that will not die. It does not help at all, and only if implemented poorly, can cause significant harm/damage to amps... sooo... either don't use one ever OR make sure if you must, you do it right so as to not harm the amp.

Side note in college I studied civil engineering and fracture mechanics is one of the areas the hit up on lightly but I had mechanical engineers that knew it well and would tell me about how this type of mechanism works... Likely the amps you see with what appears to be a tube dying early can be attributed to one of two things:

1. a manufacturing defect or defect in the raw material itself of a coating/filament/etc.
2. The user may have not used the tube 'for long' but tends to turn the amp on/off a lot and is giving it a lot of those hot/cold cycles that cause longer term stress on the metals.

Just a WAG though. But an educated one.

~Phil
~Phil

[guitom]

So, it turned out the power transformer was bad. I've got a used PT from I don't know where. It reads 670vac across the HT leads (schematic for Ampeg G15 says 720vac). it's set up for a 5volt rectifier which i won't use. It's a gray colored casing with CS 5779 number on it. also has the numbers 5001 1. It's slightly larger than the Ampeg one but the mounting holes will line up. No center tap for the 6volt.
Can I use it? Is the 670v vs 720v a problem? There's no dropping resistor before the plate hook up.
also, I thought i might install a 6volt pilot light and do away with the one that's in there, which I'm not familiar with. i'm not planning on using the standby switch and have installed a 3 prong cord.
Appreciate any help with this.
thanks,
Tom

[Stevem]

The main concern is that you don't know if the filament winding can provide enough current to power all of the amps tubes.

I would first clip lead that one winding into the amp and. Confirm that you still have at least 6.1 volts once all the filaments are warmed up

[martin manning]

It will probably work. The high voltage difference isn't significant, and a similar core size means the current capability is likely to be similar. No guarantees, though, unless you can get its specs. Steve has a good suggestion to check the heater winding current capacity, but use short leads, and get the unloaded voltage to compare the loaded voltage to. Look for a drop of 5% or less. If you use it, you will need to add an artificial center tap, 2x 100 ohm resistors. My preference would be to keep the existing standby and pilot lamp.

[guitom]

i ran the heaters on the 7 tubes. The voltage unloaded was 6.99v ac. With all tubes heated 6.55v ac, a drop of 6.3%. The total current draw for the tubes (per the manual) is 3.1A. (Kinda high?)
is there a good source for info on this amp? A good source for power transformer?
Thanks,
Tom

[martin manning]

Hmm. Might be ok, you could wire it in, let it run for a while, and see how hot it gets. This is the replacement, I believe: https://www.fliptops.net/catalog/p-1009 ... 7591-tubes

[Stevem]

A company called Fliptops has the exact replacement PT you need.

[guitom]

So I ran just the heater voltage, nothing else. The 7 tubes all glowed, transformer didn't get even warm. I got a current reading of 1 mA. Do I need to hook up the B+ as well for this test?

[sluckey]

So I ran just the heater voltage, nothing else. The 7 tubes all glowed, transformer didn't get even warm. I got a current reading of 1 mA.

Tell us exactly how you got a 1mA reading. What meter? What meter settings? Probes connected where?

[guitom]

I hooked up my Fluke 87 multimeter to read amperage (mA, A). Leads, Red to 6v wire from transformer. Black to pin 2, 7591 tube. other 6v transformer wire to pin 7, 7591 tube.

[martin manning]

So I ran just the heater voltage, nothing else. The 7 tubes all glowed, transformer didn't get even warm. I got a current reading of 1 mA. Do I need to hook up the B+ as well for this test?

I don't understand exactly what you did to measure the current (1mA makes no sense), but you need to place the meter in series with the heater winding and set it to measure AC current in amps. Looking at the G15 (GII) schematic (3x 12AX7@0.3, 1x 6CG7@0.6, 1x 7199@0.45, and 2x 7591@0.8), I get a total of 3.55A. Be aware that it will be 2-3 times that on a cold start-up. Your meter should be ok with that on the 10A range, even if it goes over for a few seconds. IMO it's pretty safe to assume that the heater current draw is the sum of the individual rated heater current for all the tubes,

Connecting the high voltage circuit will produce more heat in the transformer, but so far this is a good result. How long did you let it run? I think an hour or so would be required to get a good idea of it. I'd go ahead and wire it in, play it for a while, and see what happens.