Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dunner84 on December 22, 2016, 06:53:55 pm
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Hello,
Last year I started building a single channel AB763 without a tremolo circuit. I started working from my own layout which I designed based on the hoffman one. I was already pretty far into the project before I discovered Sluckey built almost the same amp. His layout happens to be very similar to mine, so I used it as a reference for my build.
I don't have a 5AR4 on hand so I rigged up a SS rectifier to do y initial testing before my tube arrives. I realize that the voltages are all going to be higher, but I am thinking they are too high.
The amp starts with no issues - all the tubes glow - no hum.
With a guitar plugged in, I get no output; however, I can hear the signal in V2.
When I tap the pins of the 6l6s in V5 and V6 I do hear output.
Hear are my voltages:
B+ 490
V1:
2 - 2.3
4 - 299
9 - 299
V2:
2 - 8.9
4 - 485
6 - 8.9
9 - 486
V3:
2 - 2.5
4 - 302
9 - 308
V4:
2 - 450
3 - 407
4 - 456
7 - 450
8 - 390
9 - 450
V5:
1 - -27
3 - 491
4 - 467
5 - .2
V6:
1 - -27
3 - 491
4 - 467
5 - .2
V7:
2 - 490
Here is a link to Sluckey's schematic and layout.
I have not had a chance to really troubleshoot anything, but I did go through the amp, and I cross referenced everything with the layout.
I did not write down my voltage drops at each node, but I will do that later when I can get back to it.
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You have problems with all tube voltages except V7. They make little sense. Please recheck.
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I had a chance to scour over the amp and verified things against the schematic and the layout, and I found a few issues. I had missed a jumper on the board, and my reverb dwell pot was wired incorrectly. I fixed that up and the tube voltages are making more sense; however, there is still no output.
Here are the new voltages.
B+ 480
V1:
1 - 278
2 - 0
3 - 2.1
6 - 278
7 - 0
8 - 2.1
V2:
1 - 466
2 - 0
3 - 8.5
6 - 466
7 - 0
8 - 8.2
V3:
1 - 276
2 - 0
3 - 2.1
6 - 273
7 - 0
8 - 0
V4:
1 - 236
2 - 57
3 - 100
6 - 244
7 - 65
8 - 95
V5:
1 - -27
3 - 475
4 - 450
5 - -27
6 - 475
8 - 70
V6:
1 - -27
3 - 475
4 - 450
5 - -27
6 - 475
8 - 70
V7:
2 - 480
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You say V3 pin 3 is 2.1V but pin 8 is 0V. Did you forget the jumper between those pins on the socket? That would certainly kill all sound.
Otherwise, voltages look much better. Except... What are the real voltages for pin 8 of the output tubes? I know 70V aint right. I suspect that's really 70mV? If so, you are cooking those 6L6s! Adjust the bias pot until you have about -45 to -50v on pin 5 of each 6L6. -27v is way too low for a 6L6.
PS... We need to see a schematic of your amp.
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Thank for the reply. the jumper is in place. I am using the schematic you have posted for your TDR. I have built the same amp with the addition of NFB switch, dwell control, and I don't have a 50v bias tap. On the bias though, I can't get it any more negative than -27v.
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the jumper is in place
Crap! I was hoping that was it. So, how do you account for 2.1v on one pin and 0v on the other? Mistake writing it down maybe?
On the bias though, I can't get it any more negative than -27v.
That's a bigger issue. Fix that first then come back to the no signal problem. Pull the 6L6s until this is resolved.
I assume you are getting AC for the bias circuit from one end of the HT winding, probably about 330VAC? There should be a big resistor just before the cathode of the diode. Maybe 220K or 100K. Make that value smaller to increase the -27V. I would set the bias pot to the mid position then start experimenting with resistors until you get one that gives about -45V at pin 5 of the 6L6s. Hopefully this will allow you to swing between approx. -40v to -55v. You can use 1/2W resistors to experiment but I'd use a 2 or 3W for the permanent fix.
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Thanks again for the quick response. I rechecked that voltage, and it was indeed a writing error.
There is currently 100k resistor off my HV winding. I will see what I have on hand, and try to get the bias voltage straightened out.
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The next smallest resistor I have in that wattage is 22k. I had another 100k, so I threw it in parallel, and I have the bias voltage sitting -52v max, so I still have some playing to get a more ideal range, but I think it will work for now. The pin 8 voltage on the 6l6s is still around 90v. I am positive the meter is in volts not mV.
Still no output; however, I can hear sound from the speaker when I probe pin 1 of V1 now, just not pin 2, so maybe there is a problem with the input jacks? I have gone over them a dozen times... :dontknow: perhaps I need to take a break and come back to it later.
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so maybe there is a problem with the input jacks?
Sometimes it's confusing to wire the Fender HI/LO jacks. You could always just use a single jack to see if you get sound.
Here's a quick and easy way to verify that your jacks are wired correctly...
Connect your ohm meter to V1 pin2 and chassis.
1. With no plug in either jack, resistance should read about 34KΩ.
2. Insert a plug (may use a guitar cable but don't connect a guitar) into the "HIGH" jack. Resistance should read about 1MΩ.
3. Now insert the plug into the "LOW" jack. Resistance should read about 68K.
If any of these 3 resistance readings are incorrect, the jacks are not wired correctly.
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The pin 8 voltage on the 6l6s is still around 90v
That's impossible if you have a 1Ω resistor between pin 8 and ground. That would mean you have 90 amps of current flowing through those tubes! Your whole workbench would be on fire. :icon_biggrin:
What make and model meter do you have?
Oh wait a minute! Do you even have anything connected to pin 8? My board layout does not show anything on pin 8. That would explain a very high voltage and also explain no sound.
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The meter is a fluke 115. And I have nothing connected to pin 8.......... :BangHead:
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I have the two 1 ohm resistors to ground just sitting there happily all by themselves.
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About the bias voltage... If you copied my circuit you have a 10K bias pot and one end of the pot connects through a 10K resistor to ground. Sound familiar? If so, change that 10K resistor to a 27K resistor like the super reverb uses. That'll bump the negative bias voltage up considerably.
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Thanks for your help. I really appreciate it. The amp has come to life! I suspect the bias is still too hot, so tomorrow I will make the changes to the bias circuit you suggested, and dial it in. Other than a bit of fizz and mild hum it sounds great. I think that correcting the bias will clean that up. I will also give the reverb a wurl tomorrow as well.
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Are there any good ideas where I can find a good DC source to elevate my heaters? Normally I would use pin 8; however, this amp had a fixed bias, so that doesn't work in this case.
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Sure. Use one of the B+ nodes. Node C will be a good choice. It's well filtered.
Now build a simple 2 resistor voltage divider like this. The values I used in this example will provide 45V. Change resistor values to obtain a different voltage.
I'll be surprised if elevating the heaters will be beneficial in this amp.
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Thanks for the circuit. You may be right about not needing it. It is something I can try once this is all working properly.
I swapped the 10k resistor for a 22k which made me able to adjust it between -65 and -82v, so I bumped the HV tap back to 100k from 50k.
Here are the current symptoms:
I get output but its a bit weak and very distorted. I am working with 4 6l6 test tubes. 1 is a bad tube for sure 1 is a good tube for sure. My pin 1 measurements are as follows 33vdc on V6 and .1Vdc on V5, and they stay like that regardless of which tubes I have in place. My pin 1 to ground resistors are 10r not 1r
I have NFB switch that lifts the NFB, and there is no discernable difference in the tone or output level when I switch it.
With no reverb tank installed, I get a loud squeal when I turn the reverb past 7. With the tank installed this does not happen. I do hear reverb but it is faint.
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My pin 1 measurements are as follows 33vdc on V6 and .1Vdc on V5, and they stay like that regardless of which tubes I have in place. My pin 1 to ground resistors are 10r not 1r
Pin 1 is not used in a 6V6 or 6L6. Pin 8 is the cathode and that is the pin you should have your 10Ω resistors connected.
If you followed my layout you have a 1500Ω resistor connected between pin 1 and pin 5. If that's your case there should be about -45v on pin 1 when you get the bias supply sorted out.
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Sorry, I really meant pin 8... The 1500r resistor is on pin 1. 10r resistors to pin 8
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I hope everyone has had a great Christmas!
I've made a bit more headway, but I am still a little stuck.
I traced some of the issues I was having to bad power tubes. I also replaced the 10r resistors with 1r to make bias reading easier. I threw in a good set of jj6V6s, and all of the noise issues went away. with 465v on the plate, the hottest I can bias them is to 7w, so about 50% which is rather cold. My initial plan for this amp was to be able to switch between 6v6 and 6l6 tubes, but so far, I can't get the bias range right for either. My local electronics shop is open tomorrow, and I will pick up a few resistors to replace the 100k HV Tap to sort that out. This isn't the issue though.
Here are my current symptoms:
- guitar is distorted even at the lowest volume setting. The preamp pots all work as they should, but the overall signal is a little thin, and distorted/fizzy.
- reverb is very faint. I can hear the reverb tank loudly if the springs get jostled, but barely anything otherwise.
- amp freaks out a bit when the reverb is turned all the way up.
- no discernible change when I flip the NFB lift switch.
I have checked, rechecked, re re re re checked the layout against the schematic, and everything is looking ok. I am not too sure where to go, possible pre-amp tubes?
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I would think the cold bias is what's making hte tone a bit off. If you can get that up a bit higher near 70% you'll do better. If you want to swap out between 6V6 and 6L6 you may need two separate bias resistors that are switchable, or a larger sweep bias pot, but not sure.
~Phil
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I was thinking that, but I'm not sure that is a symptom of a cold bias. The other thing is that the same issue was present with the 6l6's, and they were biased appropriately... The presence of the other symptoms leads me to believe there is another issue.
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Have you checked the values of all the resistors installed?
Measure them or use the colour code to identify the value, write the value in the schematic one by one
Have you measured continuity in the circuit of all the parts , all the wires, all the Ground bards, all the wires from tube to circuit?
Take the schematic and measure continuity of everything, beginning to end
Are you sure all the tubes you are using are Good?
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I have gone through and checked continuity and resistor values vs the schematic, and also done the math on the parralelled resistors. I have diodes in the rectifier position, and known good power tubes. The 12ax7s are good as they were verified in another amp. I dont have a means of checking the 12at7s, but I have 4 NOS tubes that I have never used, but purchased from a trusted source.
Is there an easy way of bypassing the reverb circuit and eliminating it as part of the problem?
*Edit* If I lift the top leg of the 500p cap, will that remove the reverb portion of the circuit?
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If you just shut the reverb volume to 0 it should do the same as removing it from the path, unless you've got a bad reverb pot. You could also just disconnect the output at R21 so it doesn't go up to the next stage I'd think.
~Phil
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I have gone through and verified all of the components values/integrity, and I have now verified all the tubes I am using, and reflowed all of the solder connections again. I even removed the diodes and plugged in a 5Y3. The bias is still quite cold on the 6v6's, but that should not be causing this. The volume does not get over voice level, and it sounds like I have an overdrive pedal plugged in.
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How much voltage on pin 5 of the 6V6s?
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Pin 5 is sitting at -40.6V
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Drop it down to just -36V. Any better?
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The sweep range is -40 to -58v. This is because I changed the HV tap from 100k to 68k, and the 10k resistor to 22k to get things setup for 6l6's.
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The sweep range is -40 to -58v. This is because I changed the HV tap from 100k to 68k, and the 10k resistor to 22k to get things setup for 6l6's.
But you ain't using 6L6s any more.
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You are very correct, do you have a suggestion for how to achieve a large enough sweep to accommodate both tube types? Or, what size HV tap resistor will get me in a good range for the 6V6. I just recalculated the dissipation and its only 2.2 watts! way too low, but I don't think it is the reason for this, but maybe it is?
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I replaced the 68k resistor with the 100k resistor, and can dial the - voltage down; however, it has no affect on the current issue. :w2:
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do you have a suggestion for how to achieve a large enough sweep to accommodate both tube types?
Yes. Make the bias pot larger than the resistor connected to ground. I'd try a 25K pot and a 10K resistor, then juggle the resistor as needed.
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The amp has come to life! I suspect the bias is still too hot, so tomorrow I will make the changes to the bias circuit you suggested, and dial it in. Other than a bit of fizz and mild hum it sounds great.
So, what has changed since you said that?
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That was me being a little hot off the line. I had the volume down low because the kids were sleeping. The distortion and volume issue were there, but I could not hear at such a low volume.. It is kind of strange, at the lowest volume setting you can here the clean chime, but there is some light distortion on it. As soon as you pass that initial-on-volume, it dirties right up and that sweet character disappears.
I was reading through your build, and I got excited when you had a similar issue due to the tremolo loading, but then my heart sank when I noticed the 47k to ground...
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I didn't realize that not having the reverb tank plugged in could damage the reverb transformer :dontknow:, so I may have damages it. I disconnected it from the send jack, and measured it. It comes in around 4ohms, which may be OK since it's a small winding, but I can't say for certain. I reattached the reverb transformer to the send jack, pulled V2, and jumpered over the 3.3m resistor, and the the noise floor of the amp shot right up, the issue is still there and is significantly worse. I can hear almost a motorboating type tone in the distortion now.
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I didn't realize that not having the reverb tank plugged in could damage the reverb transformer :dontknow: , so I may have damages it.
Disconnect the Reverb transformer leads, measure DC resistance of the Primary and the secondary
Primary should be around 2K
Secondary should around 1ohm
While troubleshooting,I have measured in the past vintage Fender original transformers from the Blackface/Silverface Reverb amps family with 1K on the primary and other with 30ohms on the secondary, the reverb still worked well and sounded good and the problems the amplifiers had were not related to the reverb transformer.
But as a simple test use the values I showed you and report back
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Thank you for the reverb transformer info, when I get this figured out, I will take a look at it more closely.
With the 3.3m resistor jumped the noise floor went through the roof. I pulled one tube at a time starting at V1, and V3 is the culprit. I rolled a few tubes through, but it seems to be the location, and not the tubes. I took voltage readings again, and I noticed there is 4mV on the grids pin 2 and 7. I am pretty sure that should be 0, but I am not sure what kind of tolerances are ok. I will go over the socket and connections again to see if I missed anything. I have a good fluke meter, but it doesn't measure capacitors very well, and I suspect the voltages on pins 2 and 7 is due to a leaky one somewhere.
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I suspect the voltages on pins 2 and 7 is due to a leaky one somewhere.
Why?
Did you use old parts re-used parts when building the amp?
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and I noticed there is 4mV on the grids pin 2 and 7. I am pretty sure that should be 0
4mV is zero in a tube amp.
Don't jumper the 3.3M.
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Ok, so I will ignore that voltage while I troubleshoot this. Is it safe to say my problem is with a component connected to V3?
All the components are new expect for the PT and choke which came out of a 1950s organ. The caps are all new orange drops, and the resistors are all new 1/2 watt metal and carbon film. Pots are the mini alpha pots, and the switches are new carling. It's not a big stretch that new components can be bad, or maybe the heat sink didn't do its job during soldering.
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The caps are all new orange drops
So I don't see any reason to think it's a Leaky cap, you had to be really really unlucky
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I have been pouring over these connections, and I am still not much closer. There is no DC on any of the pots, or on the input, and I don't see any AC on the rectifier output.
With the volume cranked right up and no guitar plugged in, I am hearing intermittent swirling.. It is only intermittent though. I also hear a very faint very low frequency pulse on startup, but only at startup. I am not sure that this means anything. With a guitar cable plugged in, the noise floor jumps up a bit, and I am picking up a radio station. That is a first for my house. It happens to my friend all the time, and he is a neighbourhood over. Again, I don't think that is related to my issue. It is just an observation.
I am wondering if there are any key voltages on the board that I could be looking at to help narrow this down.
I still also think it is odd that the NFB switch has no discernible affect on the output/tone.
It is also worth noting that I do not know the exact specs for the choke. It is a good sized choke, and it measured about 175 ohms. It came out of the same amp as the power transformer, and the circuit used the same rectifier tube, and power tubes, so it should be ok in this amp.
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With a guitar cable plugged in, the noise floor jumps up a bit, and I am picking up a radio station.
Picking up radio stations is normally solved by increasing the grid stopper resistor value of the V1 tube.
What value are you using?
I read your thread from the start again, and I got really confused, you posted the voltages 2 times and they were wrong or had writing errors, so with misinformation is also hard to help you out.
My advise:
- Do a present voltage chart, without errors or writing mistakes
- Do a video with sound of the actual problems
- post high-res pictures of your circuit and board (you can use photobucket to host the pics)
- Post an exact schematic of your circuit, you can use the schematic you based on and change in the schematic the parts that you changed in your build or the mods you might have done, that schematic has to reflect exactly the circuit you built
- if you used parts from another amp, tell which amp was that, tell the parts specs
Organize all your info, be precise
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I agree with whoops. It's time for you to show us your exact layout (don't just say it's similar to mine) and some hi-rez pics so we can compare your actual build to your layout. It's entirely possible that there is an error on your layout and even if your build completely agrees with the layout, the amp will never perform any better than it does on paper. The pics need enough detail to see values of components and how those components connect. Need to be able to follow a wire from a tube socket to the board. Need to see the wiring for the input jacks.
The fact that you can get a loud squeal by turning the reverb pot up indicates that the power amp is probably working to some degree. If so, it would be logical to look for problems with input jacks, V1, and V3 circuits.
You say the PT and choke came from an organ amp? Which one? Model number?
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You guys are completely right. I read back through everything, and it is all scatter brained. This build in general was a bit rushed, and untidy. I blame kids and holidays :dontknow:
I will be able to post everything properly tomorrow, and I may even have my tubes by then.
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I have not had a lot of time the last few days to work on this, but while was taking photos yesterday, I noticed the V1 and vV3 cathode bypass caps look bulged on the + end. It is really hard to tell because of the angle. The plastic that folds over towards the leg has separated, so I can't tell if it is bulging or if the plastic has come away. Unfortunately, I can't upload pictures right now because I am stuck at work all night, but I thought I would make note of that. I am also not sure what affect that could have on the amp. I know what completely lifting the V1 bypass cap sounds like, and that is it. It could be nothing, but it could be the cause of this issue?
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For what K bypass caps cost just swap them out with new 1's and be done with it.
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That's the plan. But it is an interesting point you bring up. For most of the members on the forum that is the case. My local supplier is terrible, and shipping from mouser is $20 to Canada, Digikey is decent at only $8, but its not exactly fast. For one off parts, it's actually a fair bit more expensive and time consuming. Sorry for the mini-rant. It is a source of frustration for me.
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That's the plan. But it is an interesting point you bring up. For most of the members on the forum that is the case. My local supplier is terrible, and shipping from mouser is $20 to Canada, Digikey is decent at only $8, but its not exactly fast. For one off parts, it's actually a fair bit more expensive and time consuming. Sorry for the mini-rant. It is a source of frustration for me.
Where are you located?
You can get a part that works from your local supplier and then in the future when you do an online order from a better supplier you add the "better" part you want.
Just make it work with what you can have now.
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I live in Victoria. I travel a fair bit for work, and I usually pick up parts I need when I'm away. Halifax has a fantastic parts store, so I stocked up on a few bigger parts when I was there. I was also just in Boston for a few weeks, and I was able to stock up on filter caps, turrets, and grill cloth.
I pulled the bypass caps, and I clipped in some spare filter caps. Problem is still there. That was really the only time to troubleshoot I have had the last several days. I still plan to post pictures, layout/schematic, and a few audio clips. I may not be able to do so until Wednesday.
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I have attached the schematic and layout I used for the build. My new voltages are included in the table in the schematic. I took two soundclips as well. One is of the distortion issue. The volume is at 1.5, and you can hear how the amp sounds ok when the guitar is played gently, and the nasty distortion comes in when played moderately. The other is of the noisefloor of the amp with V2 and the reverb tank installed. It is much noisier with them.
SoundClips:
https://drive.google.com/open?id=0B2gStVV-5NtrVGdtV2hnTlFUNUE
I will post gut shots shortly.
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Put the volts near the pins. Also B+ node volts.
It looks wrong both sides of V3. Too-too-small drop on the plate resistor, yet a not-wrong cathode voltage.
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Voltages with diode rectifier.
B+ Nodes:
A - 481
B - 479
C - 466
D - 432
Measured resistance is 830 Ohms
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??? Are you talking a different setup and all new voltages?
LOOK at what I sketched for your V3 voltages. Plate should NOT be 92% or 284/308 (or whatever it is now). Plate should be, for Fendery stuff, 70% of the node B+. Also the currents should add-up.
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The setup hasn't changed. I used the diode rectifier when I measured the voltages for the chart. All of them were taken with respect to ground.
Edit- sorry, I see where the confusion is. I did not notice the node voltages on the schematic when I was updating it. The main voltage chart is correct, but the b+nodes labelled on the schematic are not. I apologise, I did not notice them there until now.
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I dialed in my tone generator to 100mVac and traced it through the circuit today, and had some interesting findings. First of all, the tone came through very loud and clear. Second, the voltages were odd. I had 2.7Vac out of V1A, 11Vac out of V1B, 18Vac out of V3 and over 110Vac out of the phase inverter!! I double checked those voltages, and they are correct. Those are also with the volume control turned all the way up.
This is also with V2 pulled and the reverb tank disconnected.
Is the average amplitude of a guitar much higher than 100mV? If it is, that would explain the volume is so overdriven, but I am not sure how that would relate to low output volume.
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Is the average amplitude of a guitar much higher than 100mV? If it is, that would explain the volume is so overdriven, but I am not sure how that would relate to low output volume.
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Is the average amplitude
"my" standard is typically 50mVrms.
make a cheater n plug your guitar into the scope, hammer 1 note, softly brush your tips over a string, divide by 2 :laugh:
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These amps full-up should clip with 20mV input.
I've lost track of your tubes, but if 110V at PI becomes 22V, that's pretty-near clipping for the grids of most power bottles.
100mV-200mV is more typical playing, but either you turn-down some or you want some clipping.
If you have that much inside the amp but it isn't screaming as it should, you have bad contact or bad parts in the final stage from big bottles to loudspeaker.
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I injected my 100mV tone, and adjusted the volume on the amp until there was 10Vac coming out of the PI. With 10V at the output of the PI, I get 69Vac and 79Vac at the plates of the 6v6s and 135Vac between them.
There is something odd though. My 100mV signal comes through loud and clear. It's crystal clear and annoyingly loud when set with 10V leaving the PI.
I plug in my guitar, and low distorted output. I measured the output of the guitar at the input jack and it varied between 40mV and 300mV. Regardless of the amplitude, the amp sounded the same.
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Have you checked to see whether any of the coupling caps are leaking DC? That can result in the symptoms you describe.
Also check that the ground side of each filter cap is electrically connected to the ground return. If the pre-amp cap ground is not connecting, it can give you funny voltages and cause unwanted coupling between the pre-amp gain stages through the power rail, which results in mushy distortion
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I have checked for leaking DC and there is none that I can find. All of the filter caps are connected to the same ground point at the power transformer, and the connections are all sound. This is a weird one. I will definitely keep at though and report back any new findings, and hopefully a solution!
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I have checked for leaking DC and there is none that I can find.
How did you check?
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I didn't see anything in his voltage readings that would indicate leaky coupling caps.
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I didn't see anything in his voltage readings that would indicate leaky coupling caps.
He hasn't reported any DC voltages on the AC-side of the coupling caps. Even 1V will throw off the bias at the grid of the following pre-amp stage, and reduce gain.
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I have checked for leaking DC and there is none that I can find. All of the filter caps are connected to the same ground point at the power transformer, and the connections are all sound. This is a weird one. I will definitely keep at though and report back any new findings, and hopefully a solution!
Hi dunner. Did you test the caps like Fig4-4 (P57) of the attached chapter shows?
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> He hasn't reported any DC voltages on the AC-side of the coupling caps. Even 1V will throw off the bias at the grid of the following pre-amp stage, and reduce gain.
The thrown-off bias will usually be seen in the plate voltage of the next stage. We would expect say 170V and find maybe 100V. I have not looked, but I trust Sluckey would have hollered if he saw that.
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I had more time today, and I made a big discovery. My power tube screen resistors are not 470ohms...... They are 470k. I am not sure if that is what is causing my issue, but it could be, and I want to swap then out. I don't have any larger wattage 470r on hand, but I do have a bunch of 1K 1/2 watt that I can parallel. Would 1 watt suffice for testing purposes? I think the current resistors are 3 watters.
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You'll be fine using two 1/2 watt 1Ks paralleled for testing. Fender used 470Ω/1W on most all those blackface amps.
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I swapped in some 820s in parallel and IT WORKS!!! The nasty distortion is gone, and the amp sounds as great as it can through the test speaker on the bench. I don't want to know how many times I checked those resistors. On the bright side, I learned a tremendous amount, and it is mostly thanks to all of the help I received on these forums, so thank you!!! It is amazing how much more I learn after each build.
The voltages all look really good now. The jj6v6s are sitting with 448 on the plates, -39 at the grids, and 30mA flowing at the cathode which is as cold as I can bias them. I will be tweaking the bias circuit a little more, but I have some NOS tungsol 5881s that I am thinking of throwing in. The voltages will also drop a bit once I replace the diodes with the 5ar4.
The last issue to sort out is the reverb. It comes through very loud and clear when I tap the tank, but is very weak when coming through the signal path. It also kills the volume of the amp when I turn the reverb passed 5, and causes the power tubes to glow blue.. I think I will remove the dwell pot, go to the stock configuration, and proceed from there. In the meantime, I have a functional amp that I can finally enjoy, and start to build a cab for.
Thank you again for all the help. This was a weird project for me, and I really appreciate everyone putting up with how scatter brained the whole thing was.
I will post pictures when it is all finished, and will probably be back before then if I can't figure out the reverb.
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The last issue to sort out is the reverb. It comes through very loud and clear when I tap the tank, but is very weak when coming through the signal path. It also kills the volume of the amp when I turn the reverb passed 5, and causes the power tubes to glow blue.. I think I will remove the dwell pot, go to the stock configuration, and proceed from there. In the meantime, I have a functional amp that I can finally enjoy, and start to build a cab for...
What plate and cathode voltages have you got on the reverb driver tube? (And did you test the coupling cap going to the reverb driver for DC leakiness?)
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V2 Voltages
1 - 435
2 - 40mV
3 - 7.9
4 -
5 -
6 - 447
7 - 40mV
8 - 8
It looks like there is about 40mV leaking on the grids. I have heard/read conflicting info on the allowable voltage on the grids. Also, it doesn't look like the reverb driver has a coupling cap. It is tied directly to the reverb transformer.
It is possible I damaged the reverb transformer when I first started my testing. This was my first reverb build, and I did not know the tank and reverb driver needed to be disconnected. When I checked the winding resistance, I remember the values looking right, but there is always a chance it is not good. In my experience, the Hammond transformers are pretty robust though.
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... Also, it doesn't look like the reverb driver has a coupling cap. ...
There will be something like a 500pF coupling cap in front of the reverb driver grid ('C22' on the Weber 6A40 schematic)
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I have not checked the 500p cap for leaking DC yet, but I am starting to suspect it, so I will pick up a new cap on Monday when I hit the local shop.
I had a chance to work on things tonight. I tweaked the bias circuit to get the 6v6s around 70%, and I also removed the dwell pot, and replaced it with the 1m resistor.
The reverb is now coming through ok until about 3.5 on the dial. After that, the overall sound of the amp slowly dies off, and the bias of the power tubes drifts from -40v and 22mA to -70v and about 40mA and start to glow blue. I am going through the schematic right now to see the correlation, but it seems like it could be DC throwing things off down the line.
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You'll be fine using two 1/2 watt 1Ks paralleled for testing. Fender used 470Ω/1W on most all those blackface amps.
Fender did always use 1W screen resistors, but I'm wondering if that wasn't because maybe Leo got a shitload of 1W 470R at a good price. There seems to be very minimal current used on the screens. Seems to me 1/4W would be sufficient.
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Screen current can be quite large in OVERdrive. Not easy to predict from normal amp experience. I bet he thought 1/2W was plenty, and they kept coming back blown, so he went 1W.
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Hi dunner84 - did you ever post a schematic of your amp? (In case I missed it)
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I did post a schematic. Disregard the node voltages, and bias voltage printed on the schem. Only the tube voltages are accurate. I have also replaced the dwell control with a 1m resistor.
I have replaced the 500pF cap, and it did not correct the issue. With the exception of some pre-amp noise, which I suspect is a noisy tube, the reverb is the last thing to fix.
With the tank and driver removed, the amp squeals if I turn the reverb up past 7.
With the tank and driver installed, the reverb works and sounds nice up to around 5, after that it drops the output of the amp, and cranks up the bias on the power tubes.