Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Cree on April 26, 2020, 05:24:42 pm
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Have been tweaking this off and on for a few Saturdays now. . The switches are for testing different cathode caps and their different tones. But right now they are as listed in the schematic. Whats is going on with her as built is,, a very loud flubby bass signal above 3 on the vol control. If kept below that, and single coils are used, it plays or sounds very good. And when using my Reverb pedal I have to turn the depth way down or I have way to much reverb, so I am proud of that. It sounds good with out reverb. Sounds good but can't turn the darn thing up. I tried a 40uf in at the c1 and it blew my fuse. (may have been a bad cap, not sure) Here is the schematic I used. Remember It is for testing and I thought It was simple enough not to cause me any problems. BUT,,, here I am in need of advice.
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scope is real handy here.
what's the vdc at the top of R15
swap C9 for a .001uf and see if it went the right way.
how many speakers have you tried?
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scope is real handy here. >>> no scope here. At this present time. "Short Wave" man down the street has one but will not answer my calls.
what's the vdc at the top of R15 >>> there is a normal range of 7.83 to 8.64 vdc depends on Volume setting, and switch setting, note has been as high as 12.43vdc with all switches on and volume at 75%<
swap C9 for a .001uf and see if it went the right way. >>> I will, and I was leaning that direction but wanted more knowledgeable confirmation. Thank you<
how many speakers have you tried? > I have tried a 8 inch 4 ohm ceramic which will be the main speaker for her, and a 12 inch 8 ohm. Same results. Also it seams to sound better / or "also as good" with the 4 ohm in the 8 ohm spot, but flubbyness still there. Thank you for the suggestions.
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I'd leave C7 engaged while troubleshooting, it should keep things tighter.
that vdc "equates" to basically "clean" drive, once you get bigger the AQ will distort not well from my experimenting
what's plate voltage for the AQ?
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just glanced at the schematic . a couple questions why the large 4.7meg resistor on the 1st grid when you have the 1st triode cathode biased. usually there is just a 470k to 1meg ground ref when cathode biasing. if you are trying to grid bias just ground the cathode. also having trouble following all the stuff between the 1st and second triode. maybe simplify. look at the schematic for other single ended amps, fender 5f1 champ or supro spectator or the like , they are just a few basic components and sound great. perhaps some of this "extra" circuitry is causing the flubby tone? i think simplify it and see what happens.
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This LAYOUT has a lot in common with a champ, other than the tone stack which is called a Moonlight. (All the stuff in between the triodes). It is similar to a early fender the tone stack, but I had the parts to build one not the other. there again parts on hand, reason why the grid leak is so large, the bigger the grid leak the more signal goes into the tube, right? This is a very basic amp, all the add on's (switches) are for experimenting with components for other future builds, so I know what sounds to expect if I do this or that, got to build a junker first to build the 5 stage monster later. Just did not think she would be a head ache because it is a good bit like a champ/ any other single end amp. Other than output tube which is a small 6V6.
I'd leave C7 engaged while troubleshooting, it should keep things tighter.
I tend to do that, it sounds much better with first stage cap on.
that vdc "equates" to basically "clean" drive, once you get bigger the AQ will distort not well from my experimenting
what's plate voltage for the AQ? Voltage at the plate is around 80vdc
once you get bigger the AQ will distort not well from my experimenting.
I have not heard/read this about the AQ. Did not think at this low output she would distort much anyway. Maybe rewire for other tube, I have given thought to that. Even at low volume with one of my pedals she plays sounds good, just know there is more there that I can't use yet.
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We need to see some voltages, otherwise its just stabs in the dark
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My first reaction is for the user's safety. There is no power transformer shown. Does that mean that the circuit is wired directly to a 240v AC line?
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the too early #'s looks like you're at ~2.5W idle. a 2 stage pre gain is likely swamping the PA tube.
Audio power is probably 1W, so if you make it sound pretty good on 3, that's about it without bumping B+ to 100-120vdc. I think :icon_biggrin:
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I'm wondering about the 10K 3W in the power supply that goes from +VDC supply to ground? I've used 220K to ground for a cap-bleed resistor for draining caps while unplugged, but 10K sounds like a low value. What are your voltages at preamp anodes?
Have you considered lowering either/both coupling caps. Try a 0.003 uF in one or both of your coupling caps.
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We need to see some voltages, otherwise its just stabs in the dark
Here are some real time voltages if you are interested. All voltage readings are to GND. At main receptacle ground point on chassis. With 8 ohm speaker plugged into 8 ohm port. Tone at 5, Volume at 3. Single coil guitar plugged in and strummed hard, open strings during some readings. AC to the bridge 220VAC , 280VDC out, B+1 = 150VDC, Voltage at Pin #6 of Output tube or Red wire to output trans = 147VDC, Pin 5 at output tube or brown wire from output trans = 141VDC. B+2 = 142VDC , Plate voltage at V1a, pin 1 = 68VDC, V1b, pin 6 = 77VDC. V1a, pin 3 = .7VDC, V1b pin 8 = 1.32VDC, V2 pin 2 = 7.42 VDC. Voltage going into the tone pot, with guitar strummed hard = .8VAC max, going into the Volume pot with guitar strummed hard .4VAC max. I have clip on meter probes, in case you are wandering how I did that. It did take me awhile to do all this, as I had to wait on my wife to finish drying clothes so I could plug in my amp.
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My first reaction is for the user's safety. There is no power transformer shown. Does that mean that the circuit is wired directly to a 240v AC line?
It's no fun sitting around all day with nothing to do but plug my amp into the drier socket.lol The power transformer is a Hammond T290WX same as used in a Fender stand alone reverb. 237VAC are the spec unladen. I just rounded up to 240V.
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I'm wondering about the 10K 3W in the power supply that goes from +VDC supply to ground? I've used 220K to ground for a cap-bleed resistor for draining caps while unplugged, but 10K sounds like a low value. What are your voltages at preamp anodes?
The 10k used here is also to help soak up some voltage. I was shooting for B+1 at around 190VAC.
Have you considered lowering either/both coupling caps. Try a 0.003 uF in one or both of your coupling caps.
Yes I have given thought to lowering the caps. But I am seeing now that my target voltage is off at the B+ node. Rethinking my power supply setup right now.
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280VDC out, B+1 = 150VDC, Voltage at Pin #6 of Output tube or Red wire to output trans = 147VDC, Pin 5 at output tube or brown wire from output trans = 141VDC. B+2 = 142VDC , Plate voltage at V1a, pin 1 = 68VDC, V1b, pin 6 = 77VDC. V1a, pin 3 = .7VDC, V1b pin 8 = 1.32VDC, V2 pin 2 = 7.42 VDC.
Considering that you can get 280VDC rectified, you’re throwing away a lot of juice and really limiting your output stage driving voltage.
The preamp voltages are too low to sound good IMO.
I’d move the Output stage supply back up to the highest B+ mark (and consider increasing the reservoir cap to 100uF*), and also move the preamp supply to a higher DC point.
* If the fuse blows solely because of a bigger reservoir cap, that will be inrush surge spiking the PT (which can occur with some modern grain-oriented laminations). In any event, you should have a slo-blo fuse in the mains, but if it still blows, its probably the type of steel in the PT, and you could rig a soft start switch to get around any startup surge problem.
Incidentally, what is the OT impedance ratio?
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Incidentally, what is the OT impedance ratio?
The output transformer I am using is a Classic Tone 40-18030. I am on the 5K Connection. See attached PDF.
I am looking into all of your suggestions already as I do believe I need to rework the power supply section. Just got to do it. I did try at first a 40u clipped to the first 20u blew the fuse, then just tried the 40u alone with the 20u cut on one end, that blew the fuse I then thought it was a bad cap. put back as sch. shows. Do not know about the steel thing you mention. I have to say it has been a real test. I did not think so many would offer help. I did not think I would have to defend myself with my oddball approach to my cheep test amp. I understand it's not normal, I tried to say it is for a test bed. It was with great pain I decided to open a post about it. Should not be so. Made me feel ashamed to ask. I'll never be a Wizard, PRR, Sluckey, Shooter, Tubeswell, or one that can or could build a better amp. but I'm trying, and many just jockey the forums. So thank you to all who offered real solutions. You know who you are. Thank you.
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but I'm trying
that should be the fun part :laugh:
I started in electronics by soldering a lamp cord to a speaker to hear 60hz, 48yrs latter I'm still trying :icon_biggrin:
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Dropped some 6AQ5 load lines for comparison:
1) with the Plate at 145V and screen at 147V (approx where you are now), with the 270R cathode resistor giving you about 7.5V bias voltage
2) with the plate at 278V and screen at 280V (approx where it would be if you moved the B+). Note that you need to increase the cathode resistor to 390R (minimum) to get the necessary 16.5V bias voltage with that new scenario. But you get a lot better driving voltage in the OT. This would theoretically be capable of about 4W max power, which is really only a teeny weeny* bit louder than 2W.
Edit: 3) a 3rd load line with the plate at 278, but dropping the screen to 200V, and requiring a Cathode voltage of 8V through a '190R' cathode resistor (you could use 2 x 390R in parallel to get 'close enough'). This would help with reliability on the screen grid (still keeping the 5k Ot). You'd obviously need to separate the screen supply to a another supply node to achieve this. Did some little arrows on this to show the range of clean voltage swing at the plate (about 310Vp-p) with a 16Vp-p swing at the grid
(* power:volume relationship is logarithmic, 10W is 2 x as loud as 1W, 20W is 2 x as loud a 2W etc)