Just starting my Blues Jr. III project

[sluckey]

I have a solid state amp. Would that be the same as using a tube amp?

The light bulb can't tell if it's tube, solid state or a table lamp.

Just plug it in and turn it on.

[PRR]

Yes, lamp-limiter. Better a bright bulb than a burned-down house. Any lesser burn-up is no big disaster.

[Tbone55]

It was probably a bad idea on my part honestly. If you studied the limiter like you did this amp, I have a feeling its all gonna work fine. You clearly put in a lot of effort. Sorry if I glitched your confidence.

j

Hey, no problem. Your suggestion was good. I'm going to try it on my solid state amp first. Better to have some idea of what to expect than none at all.

Thanks.

[Tbone55]

I have a solid state amp. Would that be the same as using a tube amp?

The light bulb can't tell if it's tube, solid state or a table lamp.

Just plug it in and turn it on.

I think you've got more confidence in my build than I do. 

[Tbone55]

Yes, lamp-limiter. Better a bright bulb than a burned-down house. Any lesser burn-up is no big disaster.

Totally agree!

[sluckey]

Even if your amp has a dead short across the ac power cord the amp will just look like a wall switch to the lamp limiter and the bulb will be full bright. No damage done. You don't want to see a full bright bulb. For that size amp I would recommend a 40w to 60w bulb. If you use a 200w bulb as U.Doug uses, the lamp won't even glow if the amp is good, but you can actually play the guitar through the amp (with a much strangled sound). Once you do the lamp limiter test and verify that the bulb just glows dimly, it's time to put the limiter away and plug the amp straight into the wall.

You can check voltages while plugged into a limiter but you must realize the voltages will be low and will be useless as far as troubleshooting goes. So just check voltages if you want to get an idea if the rectifier is working but don't bother making an elaborate voltage chart for the whole amp.

[sluckey]

Here's my lamp limiter with my November amp plugged in. I chose the November because it has the same topology as your amp, except for the tube rectifier. It has three 12AX7s and two EL84s and one 5Y3. I used a 25W appliance bulb and a 60W standard bulb. Notice the bulbs are nowhere near full brightness. Yours should look very similar.

[Tbone55]

Here's my lamp limiter with my November amp plugged in. I chose the November because it has the same topology as your amp, except for the tube rectifier. It has three 12AX7s and two EL84s and one 5Y3. I used a 25W appliance bulb and a 60W standard bulb. Notice the bulbs are nowhere near full brightness. Yours should look very similar.

Thanks so much for those pictures. I was going to use a 100W bulb I had but I think I've got a 40W or 60W kicking around. Haven't been able to finish my limiter as I need a female plug. I was wondering why it's being called a current limiter? Isn't it really a power limiter? Isn't the lamp creating a voltage drop so their isn't actually 120v being fed to the amp? It's a series circuit so isn't the current the same going to the amp as it is to the lamp? All I remember is E= IxR and P=ExI

[sluckey]

That's all correct. Most of the electronics world call it a current limiter though. The idea behind that is the fact that whenever you put a resistance in series with any device, you limit/lower the current that can flow through the device. Resistance is the property of a device which opposes current flow. The light bulb is being used as a resistor in this case. The smaller wattage a bulb is the higher it's resistance. Small bulb will only allow a small amount of current to flow through the amp Big bulbs allow greater current to flow.

The limiter part comes into play because the bulb will only allow so much current to flow. Think about this. A 40W bulb connected directly to 120V will only allow .3A to flow. So, even if your series connected amp is a dead short (looks like a wall switch to the light bulb) the bulb will still only allow .3A to flow.

Some examples of using current limiting resistors are neon lamps. They must have a series current limiting resistor because when they fire (light up) they become a dead short and the resistor is needed to prevent fires in the wiring. Same thing for a LED. They must have a CL resistor or other current limiting device or they will burn up when they light up. Zener diodes will always have a series resistor unless they are being used in some small signal clamp/clip circuit.

Just like a hammer, we could have named it something else but it's still a hammer!   

[Tbone55]

That's all correct. Most of the electronics world call it a current limiter though. The idea behind that is the fact that whenever you put a resistance in series with any device, you limit/lower the current that can flow through the device. Resistance is the property of a device which opposes current flow. The light bulb is being used as a resistor in this case. The smaller wattage a bulb is the higher it's resistance. Small bulb will only allow a small amount of current to flow through the amp Big bulbs allow greater current to flow.

The limiter part comes into play because the bulb will only allow so much current to flow. Think about this. A 40W bulb connected directly to 120V will only allow .3A to flow. So, even if your series connected amp is a dead short (looks like a wall switch to the light bulb) the bulb will still only allow .3A to flow.

Some examples of using current limiting resistors are neon lamps. They must have a series current limiting resistor because when they fire (light up) they become a dead short and the resistor is needed to prevent fires in the wiring. Same thing for a LED. They must have a CL resistor or other current limiting device or they will burn up when they light up. Zener diodes will always have a series resistor unless they are being used in some small signal clamp/clip circuit.

Just like a hammer, we could have named it something else but it's still a hammer!   

Thanks for the explanation. Curious minds always want to know. 

[PRR]

The basic point is that the bulb MUST be less power than your wall-wires can supply, so that IF you have made a dead-short, the house won't burn down. On that theory you "could" go a 1,000 Watt bulb, since even old wiring can support 1,800 Watts short-term.

The *incandescent* (includes "halogen") lamp is "special". Its resistance rises quickly as it gets hot. So with a small series load the load may get most of the voltage. If the series load is heavy, the filament hots-up, resistance rises, and now the lamp is taking most of the applied voltage.

No lamp will protect against all mistakes. A small PT, shorted, may still not pull enough current to excite the lamp, and will slowly burn-up. 10r (instead of 100r) 1/4W heater centering resistors will emit smoke without much lamp-light. Transistors *always* fail before fuses or lamps (part of their magic).

[Tbone55]

HELP! I got smoke!

Well, I guess I should have expected something to. go wrong. After all, what are the chances that everything would work right off the bat. Here's the situation. I have one resistor, the 2.2K/3W that is attached to the B+ and Z points that starting smoking right away and started to glow red. I didn't see any other components that looked or smelled like they were in distress. I didn't really have any time to watch the lamp as I had to turn the amp off very quickly. I tried turning the amp on again to see if I could spot any other trouble but had to turn the amp off because the resistor started to glow.

I could really use some guidance on this. Thanks.

[sluckey]

Check resistance from Z to chassis. Should be very high. I'm betting it is close to zero ohms. If so, find out why.

[jamaio]

Sorry to hear you smoked some parts. I had a few issues with my built but no smoke.

Hope you resolve the issue quickly.

[Tbone55]

Check resistance from Z to chassis. Should be very high. I'm betting it is close to zero ohms. If so, find out why.

I didn't see your post until now but I think I've found the problem. Resistor R49 (10k/3W) which should be connected between point X and point Y was connected correctly. It was connected from X to the next lug which I believe dropped  the resistance in that section of the circuit below what it should be and the voltage coming from B+ would have been a lot more than it should have been. That toasted R47 (2.2k/3W). Funny thing is now that I measure its resistance its gone up to over 4k. I imagine the internal substance that is used to create the resistance changed by being overloaded. I'm going to have to replace both of these resistors.

One other thing I found was on one of the preamp tubes (V3). I hadn't noticed that pin 3 and pin 8 have a jumper wire between them. I didn't notice this until I was going through the schematic and comparing it to the layout. I just had a connection going from pin 8 to the 820ohm resistor. I'll have to fix this as well.

Any and all feedback welcomed.

[Tbone55]

Sorry to hear you smoked some parts. I had a few issues with my built but no smoke.

Hope you resolve the issue quickly.

Yeah well, it was too much to expect it to work off the bat. Hopefully once I change a couple of resistors it will work. Unfortunately I won't if anything else got damaged until I do. Guess you can't make an omelet without breaking a few eggs. 

[sluckey]

Check resistance from Z to chassis. Should be very high. I'm betting it is close to zero ohms. If so, find out why.

Worth repeating.

[Tbone55]

Check resistance from Z to chassis. Should be very high. I'm betting it is close to zero ohms. If so, find out why.

Worth repeating.

Yes, I did that. Meter is reading like it's an open circuit. Readout is blinking a steady 1,000kohms.

[sluckey]

Remove the 2.2K that burned. Power up again and check for B+ at the + terminal of the FWB. Also check for negative voltage at pin 2 of V4 and V5. And make sure every tube is lit up.That's about all you can check with that 2.2K out of the circuit. What voltages do you have?

[Tbone55]

Remove the 2.2K that burned. Power up again and check for B+ at the + terminal of the FWB. Also check for negative voltage at pin 2 of V4 and V5. And make sure every tube is lit up.That's about all you can check with that 2.2K out of the circuit. What voltages do you have?

I had to remove not only the 2.2k resistor but also one of the 10k resistors because it was not hooked up properly. I can't reconnect it because the leads are too short now to do so. It's the resistor that goes between X and Y. Sorry, but what does FWB stand for? Can I still power up the amp with these two resistors missing? What voltages should I be checking?

Thanks.

[sluckey]

You can still check the voltages I suggested. When you pulled that 2.2K that removed all other B+ voltages in the amp. The FWB is your full wave bridge rectifier. Black square thing with four leads. Check the B+ on the positive lead of the FWB. Check the negative bias voltage on pin 2 of each EL84 and adjust the bias pot for maximum negative voltage at pin 2.
Make sure every tube is lit up. That's about all you can do until you replace that 2.2K resistor.

You say the 10K was connected wrong? Please explain. I'm looking at your completed pic and the 2.2K and both 10K resistors appear to be connected properly. What am I not seeing?

[Tbone55]

You can still check the voltages I suggested. When you pulled that 2.2K that removed all other B+ voltages in the amp. The FWB is your full wave bridge rectifier. Black square thing with four leads. Check the B+ on the positive lead of the FWB. Check the negative bias voltage on pin 2 of each EL84 and adjust the bias pot for maximum negative voltage at pin 2.
Make sure every tube is lit up. That's about all you can do until you replace that 2.2K resistor.

You say the 10K was connected wrong? Please explain. I'm looking at your completed pic and the 2.2K and both 10K resistors appear to be connected properly. What am I not seeing?

Hey thanks for your response. Had to get to bed last night so I didn't see your response until this morning. I'll check the voltages as soon as I've had some breakfast. I should be able to check the voltage at B+(DC voltage), the A/C voltage at the FWB, the filament voltages, input voltage to on/off switch and PT and the bias voltage at pin 2 of the power tubes. The voltage at B+ is also the voltage being supplied to the OT as well. If I've got any of this wrong let me know.

Now that I look at the 10K resistor I see that it was hooked up correctly. I thought it wasn't because I had soldered it to the lug where the 82K resistor is attached but that lug is connected to the other 10K resistor so the connection was ok. I thought I had to connected the one 10k resistor to the lug where the other 10K resistor was attached but that isn't necessary.

Thanks so much for your help.

Edit by sluckey… fixed quote.

[Tbone55]

One thing I noticed in your final picture is I don't see a ground connection to the negative bias, and cathode resistors ground rail. I guess it is under the board. Although I don't think not having bias would make it draw too much current, if you had no cathode connection either. Probably not the problem, but wondering if you did in fact make that connection.  Also, I would take the power tubes out untill you verify your voltages and negative bias. I have done a couple of these conversion's. It will pay off in the end. Hope you get it worked out.

Hey, thanks for the input. You'll have to forgive my ignorance as most of this is pretty new to me. The ground connection to the negative bias you asked about is a jumper wire that is under the board. It is connected to the ground side of the filter caps and to the two capacitors (47/100v) attached to the bias pot.

The cathodes of the power tubes are each connected to a 1ohm resistor and these are connected to the same line as the jumper wire above that is the ground connection for the bias pot, bias pot resistor and the two 47/100v capacitors above.

I hope I've explained myself well enough but please ask if I haven't. Thanks again.

[sluckey]

More things to verify... six underboard jumpers. Use your ohmmeter to check them. Easy to do from the top side of the board. Put a probe on each turret that is supposed to be connected. Resistance should be zero ohms.

Check resistance from each of the four big filter caps. One probe to chassis. The other probe to the positive terminal of each cap. All should be high resistance. Low readings are a problem.

[Tbone55]

More things to verify... six underboard jumpers. Use your ohmmeter to check them. Easy to do from the top side of the board. Put a probe on each turret that is supposed to be connected. Resistance should be zero ohms.

Check resistance from each of the four big filter caps. One probe to chassis. The other probe to the positive terminal of each cap. All should be high resistance. Low readings are a problem.

Thanks. Will check those as well.

[Tbone55]

More things to verify... six underboard jumpers. Use your ohmmeter to check them. Easy to do from the top side of the board. Put a probe on each turret that is supposed to be connected. Resistance should be zero ohms.

Check resistance from each of the four big filter caps. One probe to chassis. The other probe to the positive terminal of each cap. All should be high resistance. Low readings are a problem.

Thanks. Will check those as well.

Finished checking all jumper wires and filter caps. Jumper wires all 0.1 ohms, all filter caps infinite resistance. Found a lead from one of the bias caps that I hadn't trimmed down sticking out. Lifted the board a bit to get a better look. The lead does not touch the chassis or any other turret connection. The cap is right next to where the jumper lead from the filter cap ground goes to the first bias cap of the bias circuit. Not sure if I should unsolder the pot leads or tube leads and cut the cap lead that's sticking out. I don't think it's causing a problem.

Thanks.

[Tbone55]

Hey, thanks for the input. You'll have to forgive my ignorance as most of this is pretty new to me. The ground connection to the negative bias you asked about is a jumper wire that is under the board. It is connected to the ground side of the filter caps and to the two capacitors (47/100v) attached to the bias pot.

The cathodes of the power tubes are each connected to a 1ohm resistor and these are connected to the same line as the jumper wire above that is the ground connection for the bias pot, bias pot resistor and the two 47/100v capacitors above.

I hope I've explained myself well enough but please ask if I haven't. Thanks again.

Yes. Perfect. No ignorance here. Looks like a good job. Probably something simple. I had the same thing happen to me on my first push pull amp. Same resistor. Problem is, I don't remember what it was.

Have checked all of the jumper leads and they are good. The filter caps are good as well.

[jamaio]

Tony, looking at your final build picture it's hard to see the connections at the FWB, the + goes to B+, the - goes to the turret that has an under board jumper to ground, the 2 ac connect to the red wires from the PT. Is this correct?

[Tbone55]

Tony, looking at your final build picture it's hard to see the connections at the FWB, the + goes to B+, the - goes to the turret that has an under board jumper to ground, the 2 ac connect to the red wires from the PT. Is this correct?

Yes, the B+ of the FWB goes to the B+ line at 47uf/500v filter capacitor and 2.2k/3w resistor turret lug connection. The - of the FWB is connected to the turret lug that has the jumper wire going to the negative ground of the filter caps to chassis ground. The two red wires are from the PT with one lead going to each of the A/C terminals of the FWB.

I did have to extend the PT a/c leads going to the FWB because they wouldn't reach and I used the proper wire, soldered them together and then put heat shrink around the joints. Could this be doing something?

Thanks.

[jamaio]

In the picture it appeared that one ac connection also went to Z, the ac connections only go to the PT red wires.  I had to extend those wires on my build, should not be an issue.

[Tbone55]

I have looked and looked comparing to layout, and I can't find any mistakes. What about the output secondary.  Would that cause it to pull that much current that fast, or would it have to be a short on the primary side of the output? 
Will it fire up with that resistor out? If so measure voltage at pin 2 of both el84's. That resistor feeding your power rail won't affect your bias supply working. Make sure you have -v.  Then let someone else verify this, but I think you could pull the first 10k in the B+ rail, reinstall the 2.2k, to isolate the output section. Bring power up, (EL84's out) If it doesn't overcurrent the resistor, and voltages look good, then its a short or mistake in preamp section. Please let Sluckey or PRR or someone say this is okay BEFORE doing so!

I've removed the 2.2 K resistor and am starting to measure voltages. I don't have any of the tubes installed. Stay tuned for voltage readings. There is one thing I did have to fix. I had to run a jumper wire on pre-amp tube V3 from pin 3 to pin 8. I only had a connection from pin 3 to the board. I wish I understood more about the different portions of the sub circuits. I guess a little bit of knowledge is dangerous after all. 

[Tbone55]

In the picture it appeared that one ac connection also went to Z, the ac connections only go to the PT red wires.  I had to extend those wires on my build, should not be an issue.

Yikes! I think you may have found the problem! Should the a/c lead of the FWB be connected to Z? I've got it connected to Z and my PT a/c lead is connected to Z as well!  The FWB a/c lead should not be connected to Z and neither should my PT a/c lead. I'm putting an a/c voltage on top of the DC voltage that's coming from B+. No wonder I toasted the 2.2k resistor!

I checked the 2.2k resistor and it reads just a little over 4k so it hasn't open circuited. Do you think I could still use it to test out the amp?

[jamaio]

I checked the 2.2k resistor and it reads just a little over 4k so it hasn't open circuited. Do you think I could still use it to test out the amp?

I would replace that resistor, you may not get correct voltage readings.

Glad I could help.

[Tbone55]

I checked the 2.2k resistor and it reads just a little over 4k so it hasn't open circuited. Do you think I could still use it to test out the amp?

I would replace that resistor, you may not get correct voltage readings.

Glad I could help.

Second that. I see it now. That dashed wire (underboard). Is a little confusing in that it runs beside the lead of the FWB in the pic, but does not connect.

Thanks. I needed that second opinion. I'm going to leave that resistor out and measure some voltages in the meantime without the tubes in.

[jamaio]

One of the mistakes I made, I reversed the polarity of C1, once I noticed that I questioned the integrity of everything. Several members of the forum reassured me and said it would be fine to just correct the polarity without changing the cap. Is all a good learning experience and will help with details on future builds.

You will be happy once you hear your amp working.

[Tbone55]

Here are some voltage readings without the tubes installed now that I fixed the connection problem at the WFB.

B+ Voltage - 368v

Filament Voltage - 7.54v

AC Voltage at WFB - Lead 1, 156.1v, Lead 2, 116.2v (is this normal?)

PT Bias Voltage - Pin 2, V4, -29.0v, Pin 2, V5, -29.0v

Switch Voltage - Line connection, 121.6v  PT Side, 121.6v

I didn't check the voltage at the bias diode connection to the PT. It should be around 20v. I'll check that later. Next I'll install the tubes and re-measure voltages. Can't do anything else until I get a replacement 2.2k/3W resistor but I think the amp will be fine now. More to come.

[sluckey]

Put the smoked 2.2K back in and finish checking the amp. It'll work for now and wont cause any problems, but replace it soon.

[Tbone55]

Put the smoked 2.2K back in and finish checking the amp. It'll work for now and wont cause any problems, but replace it soon.

OK. I'll do that. Can't do it right now but will later today and will post back voltages again.

Thanks.

[sluckey]

POWER SUPPLY 101... When working with a FWB such as found in this amp, measure the transformer AC voltage BETWEEN the red wires, not from one red wire to chassis. There is no center tap on the HT secondary so the voltages you measured don't mean much.

[Tbone55]

Here are some voltage readings without the tubes installed now that I fixed the connection problem at the WFB.

B+ Voltage - 368v

Filament Voltage - 7.54v

AC Voltage at WFB - Lead 1, 156.1v, Lead 2, 116.2v (is this normal?)

PT Bias Voltage - Pin 2, V4, -29.0v, Pin 2, V5, -29.0v

Switch Voltage - Line connection, 121.6v  PT Side, 121.6v

I didn't check the voltage at the bias diode connection to the PT. It should be around 20v.

Yes you did. You have -29.0v on pin 2. That's good. No need to check at diode.

OK Thanks.

[Tbone55]

POWER SUPPLY 101... When working with a FWB such as found in this amp, measure the transformer AC voltage BETWEEN the red wires, not from one red wire to chassis. There is no center tap on the HT secondary so the voltages you measured don't mean much.

New voltage measurements with tubes installed and no current limiter.

B+ 314V

Bias Voltage - Here's where there's a problem

V4 -26.8v

V5 -15.1mv

Not sure why this is happening now when I was getting the same readings from both tubes before.

Filament Voltage -    6.98v

I checked the filament voltage at the tube board supply and at each tube and it was the same all the way through. All of the tube filaments glowed although it was a bit hard to see two of the pre-amp tubes (V2, V3) because they are a different make and are constructed a bit different making it difficult to see but the tubes were definitely warm.

AC Voltage to Bridge Rectifier - 248V

The amp is also squealing and the squealing gets louder when the volume control is turned up. It's not a steady squeal but more like a bouncing around squeal. I also have no output sound from the speaker I have hooked up. This doesn't bode well.

I have one other question. There were three input jacks supplied with the kit. They look the same but the only difference was that one of them had a longer threaded barrel. I realize now that I should have used the one with the longer barrel as the guitar input jack but does it really matter if used it as the output jack for the speaker connection?

Thanks everyone.

[jamaio]

The amp is also squealing and the squealing gets louder when the volume control is turned up. It's not a steady squeal but more like a bouncing around squeal. I also have no output sound from the speaker I have hooked up. This doesn't bode well.

The wires from the output transformer need to be swapped, sluckey mentioned this earlier when you asked if it mattered which one goes to which tube.

It does make a difference which OT leads connect to the power tubes in an amp such as this that has a NFB loop. Get'em wrong and the amp may squeal, howl, or just have other odd sounding symptoms. And you can't know for sure which is the right way, so it's best to not trim the leads to length until you can turn the amp on and test it out. If you need to swap them you will have enough length to do so.

If you have already trimmed them to length, don't panic. You have a 50/50 chance of having it right.  But if not there's another easy way to accomplish the same thing. We'll talk about it more when/if a swap is needed.

I have one other question. There were three input jacks supplied with the kit. They look the same but the only difference was that one of them had a longer threaded barrel. I realize now that I should have used the one with the longer barrel as the guitar input jack but does it really matter if used it as the output jack for the speaker connection?

If I remember correctly, the longer input jack was used with 2 fiber washers to isolate it from the chassis, not sure if this will make a difference now.

Wait for someone else to answer your other questions and confirm my answers.

[sluckey]

Bias Voltage - Here's where there's a problem

V4 -26.8v

V5 -15.1mv

This is priority number 1! Pull the output tubes and fix that V5 bias voltage.

Also, disconnect the feedback wire at the speaker jack. Leave it disconnected until the amp is fixed.

[Tbone55]

I figured the leads probably needed to be swapped but just wanted some confirmation. I have the piece of wire I cut off. Can I join it back so the lead will reach V4?

I used the fiber washer that came with the kit for the input jack. The barrel of the input jack is smaller that the hole so the washer helps center the jack. There was only one with the kit.

Any chance reversing the leads will solve the tube bias on V5 and the no sound output?

[Tbone55]

Bias Voltage - Here's where there's a problem

V4 -26.8v

V5 -15.1mv

This is priority number 1! Pull the output tubes and fix that V5 bias voltage.

Also, disconnect the feedback wire at the speaker jack. Leave it disconnected until the amp is fixed.

OK. Will do. Should I reverse the PT leads to V4 & V5 or wait until I get the bias problem fixed?

[sluckey]

You got bigger things to do right now. Forget the OT wires. Fix the bias. Disconnect the feedback wire. Reassess the situation.

[Tbone55]

You got bigger things to do right now. Forget the OT wires. Fix the bias. Disconnect the feedback wire. Reassess the situation.

OK. My order of operations

1. Verify previous bias voltage measurements with power tubes installed. V4 -28.6, V5 -15.4mv
2. Remove power tubes and re-measure bias voltage. V4 -28.6v, V5 - 28.6v
3. Re-installed power tubes and re-measured bias voltage. V4 -28.6v, V5  -15.2mv
4. Re-installed power tubes and measured supply voltage at pin 9. V4 242v, V5 242v
5. Measured output voltage of power tubes at pin 7. V4 361v, V5 361v
6. Removed power tubes and measured supply voltage at pin 9. V4 340v, V5 340v
7. Measured resistance value of resistors on pin 2 of each power tube. Verified value of 1.5k.
8. Measured resistance from pin 3 of power tubes to ground to verify the only resistance was the 1ohm resistor. Verified as 1ohm.

From the above I felt that there was a problem with the V5 power tube. To see if this was true I replaced it with the V4 power tube and the bias voltage read -28.6v. Looks to me like there's a problem with the power tube internally.

Would you concur?

[sluckey]

Would you concur?

Yes. But just to be sure put the V5 tube in the V4 socket and see if V4 now has the bad bias voltage reading.

[Tbone55]

Would you concur?

Yes. But just to be sure put the V5 tube in the V4 socket and see if V4 now has the bad bias voltage reading.

Ok. Did as you suggested. The bias voltage read -2.0v. Not as small as the reading when it was in the V5 position but definitely way below where it should be. By the way, the bias pot was already turned as far as it can go to begin with (it came that way) so if I turn it the other way the voltage will go smaller (less negative). I'll purchase some new power tubes tomorrow. I'll also have to buy some new pre-amp tubes as well. I shut off the lights so I could see the filaments glow and the pre-amp tube in V1 was the only one I could really see lit well. V2 wasn't noticeable and V3 I saw a very, very, dim glow. I guess I should have taken voltage readings from them but will do so in order to have a record of it.

Would you agree I have a bad power tube for sure now? By the way, I'm not getting any squealing any more. No smells, no noise, just some heat off the filter caps and power tubes which seems normal. I won't plug into a speaker until I replace the pre-amp tubes. When I did before I wasn't getting any sound at all.

Thanks.

[Tbone55]

Would you concur?

Yes. But just to be sure put the V5 tube in the V4 socket and see if V4 now has the bad bias voltage reading.

If it was an original tube from the circuit board, it was likely already cooked. Don't think it was anything you did. They love cooking tubes, which is why doing this conversion is a good thing.

Yeah, apparently they weren't biased very well. The fixed resistor in the original circuit that biased them was replaceable with a bias pot but who wants to touch these circuit boards and burn it. I was going to mod the amp with a mod kit but the more I thought about the less I wanted to do it. This is what I was really looking to do. A bit frustrating right now but patience is the key. Tomorrow I'll buy the replacement resistor I need as well as some new tubes, probably replace all of them and keep the good ones for spares.

Thanks for being here.