I see that in principle being able to adjust the bias resistor of the cathode for the same current could be a good idea, but in practice what is the benefit?
Without it, the tubes settle into their optimum operating state for the B+ provided. If there are gross imbalances, measured with a 10R resistor on the cathode, the output tubes can be rejigged or replaced. It just looks like a lot of overcomplication for no real benefit. If the tubes are imbalanced (within reasonable limits) then so what? It is only the change of grid potential that is amplified to the output transformer, and that is referenced to ground.
Without it, the tubes settle into their optimum operating state for the B+ provided. If there are gross imbalances, measured with a 10R resistor on the cathode, the output tubes can be rejigged or replaced. It just looks like a lot of overcomplication for no real benefit. If the tubes are imbalanced (within reasonable limits) then so what? It is only the change of grid potential that is amplified to the output transformer, and that is referenced to ground.
The schematic in post #1 is the ST-35 as I built it.
In #170, that is the schematic of the simpler, cheapo $400 Chinese amplifier kit that I built recently. I was looking at it - for comparison purposes only - since it uses these same output tubes, and I like it very much. Sorry for any confusion.
I'll post the current ST-35 schematic again here so people don't have to go back to post #1. For those interested in a feature summary comparing the ST-35 built here vs. the original ST-35 design, I also wrote this change list as well as a pictorial wiring diagram in addition to the schematic.
When I perform the adjustment procedure, I measure 350mV at each of the test points for the 10 ohm resistors. The bias adjustment procedure is as follows:
----------------------------------------------------------------------
Power on the amplifier. Set meter to DC Volts. Place black negative probe on board ground reference screw. Place red positive probe on terminal post V1. Adjust potentiometer R9 until you measure 0.350 VDC (350 mV).
Repeat above procedure at R10 & V2, R11 & V3, R12 & V4.
The current setting of each EL84 output tube should now be 35 mA. This can be adjusted to your preference via the on-board bias potentiometers, providing a useable range of 320 to 450 mV (32 to 45 mA).
----------------------------------------------------------------------
Very funny. 😉 I hope I have cleared up everything.
Attachments
@6A3sUMMER
I believe on post #170, @Never Get Old was illustrating by example that he likes his no name ‘chinese‘ amplifier design better than his current incarnation of the ST-35. That schematic on post #170 is the chinese amplifier. It is as you state, a completely different design and completely different from the ST-35 posted on post #1.
At least that’s my understanding of it. If you look at his voltage measurements, you’ll see that they are more in line with the ST-35 and not the chinese amplifier schematic.
So far, I feel as if he built the ST-35 properly, as long as we know he biased the output stage to 35mA as instructed. After that, there may be a chance that he wrongfully installed incorrect values of certain key parts, i.e. C1 (which can completely change the the bandwidth of the amp if too small of a cap was installed), C2, C3, along with the values associated with the feedback network, poorly performing output transformers, etc…all of which in my view, can be solved with verifying the build on an oscilloscope with a sinewave generator.
Of course gridstoppers are always a nice touch, right at the grid pin. A zobel can be useful, but it is very doubtful that he has miles of cable attached to a ruthlessly difficult speaker load; it’s a simple vifa 3 way, not a pair of Avalon’s or Thiels. Then again, an impedance and phase measurement of the speaker from 20Hz to 20khz would also be useful, much more useful than measuring the DCR with a DMM across the speaker terminals.
Finally, do understand that NGO is using very subjective terminology in his descriptions. We have no idea what ‘sub bass’ means. It’s just a term that is conjured up relative to what he has heard in his past with amps that work for him (i.e. Dynaco ST-70, chinese amp, etc…). In post #156, he states that the chines amp ”sounds greateven though the output transformers distort badly above 6 watts/ch from 20Hz to 30Hz.” The chinese amp distorts badly at 20Hz to 30Hz at 6watts? Really? Did he measure that? How did he know? It is entirely possible that it clips at 20Hz (transformer saturation for example), but a scope picture would be illustrative, no? Thankfully we do listen to this thing called music which has a 14dB crest factor compared to ‘sinewaves‘.
An amp that is bandwidth limited in the bass will always sound more “midrangey, forward, trebly”, etc…since it is devoid of bass. Then again, an amp with a high output impedance coupled to a speaker with very high impedance swings in the midrange frequencies will also sound ”forward and midrangey”. Some like that sound. Others don’t. A lot has to do with room acoustics, personal preference, etc…we know none of this info.
I would recommend NGO to systematically check every value of his ST-35 build and make sure they are the correct values. In many cases, you cannot check these values with both ends of the component soldered onto the PCB. You have to lift one end and measure the value. That’s quite a bit of work. You also did this curious thing about adding extra wiring on the pcb board since you have this moral obligation to make it like a point to point wired design (Post#133). Interesting…and fingers crossed that none of the wires got crossed by accident. Good double check.
You can hunt for the problem, properly with a scope and a sine wave generator. It’s a completely legal thing to purchase an inexpensive Rigol, learn how to use it and hunt for the issue. But you might be allergic to that as you state. You might even find that with a distortion analyzer that the ST-35 seems to have a more dominant 3rd harmonic vs 2nd harmonic compared to the other amps in your collection.
Good luck and enjoy the journey.
EDIT: It appears we have verified the requisite 35mA bias for the output tubes…that’s an excellent start. Keep it up!
Best,
Anand.
I believe on post #170, @Never Get Old was illustrating by example that he likes his no name ‘chinese‘ amplifier design better than his current incarnation of the ST-35. That schematic on post #170 is the chinese amplifier. It is as you state, a completely different design and completely different from the ST-35 posted on post #1.
At least that’s my understanding of it. If you look at his voltage measurements, you’ll see that they are more in line with the ST-35 and not the chinese amplifier schematic.
So far, I feel as if he built the ST-35 properly, as long as we know he biased the output stage to 35mA as instructed. After that, there may be a chance that he wrongfully installed incorrect values of certain key parts, i.e. C1 (which can completely change the the bandwidth of the amp if too small of a cap was installed), C2, C3, along with the values associated with the feedback network, poorly performing output transformers, etc…all of which in my view, can be solved with verifying the build on an oscilloscope with a sinewave generator.
Of course gridstoppers are always a nice touch, right at the grid pin. A zobel can be useful, but it is very doubtful that he has miles of cable attached to a ruthlessly difficult speaker load; it’s a simple vifa 3 way, not a pair of Avalon’s or Thiels. Then again, an impedance and phase measurement of the speaker from 20Hz to 20khz would also be useful, much more useful than measuring the DCR with a DMM across the speaker terminals.
Finally, do understand that NGO is using very subjective terminology in his descriptions. We have no idea what ‘sub bass’ means. It’s just a term that is conjured up relative to what he has heard in his past with amps that work for him (i.e. Dynaco ST-70, chinese amp, etc…). In post #156, he states that the chines amp ”sounds greateven though the output transformers distort badly above 6 watts/ch from 20Hz to 30Hz.” The chinese amp distorts badly at 20Hz to 30Hz at 6watts? Really? Did he measure that? How did he know? It is entirely possible that it clips at 20Hz (transformer saturation for example), but a scope picture would be illustrative, no? Thankfully we do listen to this thing called music which has a 14dB crest factor compared to ‘sinewaves‘.
An amp that is bandwidth limited in the bass will always sound more “midrangey, forward, trebly”, etc…since it is devoid of bass. Then again, an amp with a high output impedance coupled to a speaker with very high impedance swings in the midrange frequencies will also sound ”forward and midrangey”. Some like that sound. Others don’t. A lot has to do with room acoustics, personal preference, etc…we know none of this info.
I would recommend NGO to systematically check every value of his ST-35 build and make sure they are the correct values. In many cases, you cannot check these values with both ends of the component soldered onto the PCB. You have to lift one end and measure the value. That’s quite a bit of work. You also did this curious thing about adding extra wiring on the pcb board since you have this moral obligation to make it like a point to point wired design (Post#133). Interesting…and fingers crossed that none of the wires got crossed by accident. Good double check.
You can hunt for the problem, properly with a scope and a sine wave generator. It’s a completely legal thing to purchase an inexpensive Rigol, learn how to use it and hunt for the issue. But you might be allergic to that as you state. You might even find that with a distortion analyzer that the ST-35 seems to have a more dominant 3rd harmonic vs 2nd harmonic compared to the other amps in your collection.
Good luck and enjoy the journey.
EDIT: It appears we have verified the requisite 35mA bias for the output tubes…that’s an excellent start. Keep it up!
Best,
Anand.
I did not do the tests of that Chinese amplifier kit personally, but highly knowledgeable YouTube member XrayTonyB did in a video build series where he addressed the issue and tested everything as he went along. The transformers in that kit are great from 30 Hz up. From 20 Hz to 30 Hz, the wave gets really distorted above about 6 watts/channel. I posted a build thread for my Chinese amplifier and links to his test videos elsewhere on DIYaudio. I won't repost here to avoid confusion.
25 Hz to 40 Hz. Many people do not pay any attention to this range and don't have speakers that even perform down in that range.
"In this range, human hearing is less sensitive, so these notes tend to be felt more than heard. The low E-string on a bass guitar is usually tuned to 41.2 Hz, while the lowest note on a standard piano is A at 27.5 Hz." https://en.wikipedia.org/wiki/Sub-bass
I did this three times, including 1) Testing when the components arrived from Mouser and DigiKey as I unpacked each one, 2) Testing immediately before each was installed, and 3) Again visually verifying after each one was installed.
Yes, I did. The circuit boards flex so much when inserting and removing the tubes that there is a serious risk that one of the PCB traces will break. This actually has happened on the original ST-35 per owners of that amplifier. The new PCBs are high quality, but the new "Micalex" tube sockets included with the kit have a death grip on the tubes that it so severe that I cannot recommend that anyone else use them. I am concerned that over time one of the PCBs is going to snap due to the force of inserting and removing tubes, even though the circuit boards are of good quality. I never have been a fan of PCB traces anyway. With this amount of flex, I think over time a PCB trace will snap.
I am fortunate to have on hand for comparison known amplifiers against which to measure my perception: 1) The aforementioned Chinese amp kit that I built, which uses the same output tubes, and 2) Also the Dynaco ST-70 series ii that I have enjoyed since 1991. Those two sound great. The third, which is the ST-35, sounds not-so-great. As I said, this is a game of which one of these is not like the others? The room doesn't change, and hasn't changed for 20 years. The speakers don't change, and haven't for 25 years except for replacing the drivers a few times as they aged. The sources don't change and neither do the music I use to test, mostly jazz and blues if anyone is interested. The only thing that changes is the amplifier.
Correct, as described earlier. Each speaker has a pair of 8 ohm Vifa 8" drivers in parallel with each other in a very large ported enclosure. Each one also has a pair of 8 ohm Vifa 5" drivers, also wired in parallel, in an internal sealed midrange box. The crossovers for those are first order. Each tweeter is a Morel 8 ohm with a 2nd order crossover. Nothing difficult to drive in any of it. It's just a large and fairly efficient 3-way floorstanding speaker, which when pushed, will rattle the walls with bass and sub-bass, but not when using the ST-35.
I have owned a lot of amplifiers over the years, both solid state and tube, and I am placing this one second-to-last on the list. I refuse to believe that this sound is normal for the ST-35. It has harsh midrange, light bass, and light or nonexistent sub-bass. Whether we can ever figure out why may very well depend on finding someone with the equipment, knowledge, time, and willingness to test it. I'll order some different driver tubes, but I doubt that they are going to make a huge difference.
Last edited:
Good explanations for all.
For me bass is >20Hz to 50Hz. Midbass is 50Hz to about 200Hz. <20Hz is sub bass.
But this is just semantical.
Maybe this helps correlating the subjective with the objective (it’s a hobby anyway!):
BTW, your speaker sounds like a solid 4 ohm load or lower. A pair of 8 ohm woofers, wired in parallel, will have an impedance dip around 100-200Hz at 4 ohms or usually lower which is a tough load for any tube amp imho. Keep that in mind. It sounds like an MTMWW.
Do measure the impedance/phase of your speakers if you can. A Dayton Audio DATSV3 is incredibly helpful if you can borrow one. Available at Parts Express. You can always return it if you don’t like it.
The ST35 as designed might be current starved in this setup. Then again, it doesn’t completely explain why you like the chinese design either.
Best,
Anand.
For me bass is >20Hz to 50Hz. Midbass is 50Hz to about 200Hz. <20Hz is sub bass.
But this is just semantical.
Maybe this helps correlating the subjective with the objective (it’s a hobby anyway!):
BTW, your speaker sounds like a solid 4 ohm load or lower. A pair of 8 ohm woofers, wired in parallel, will have an impedance dip around 100-200Hz at 4 ohms or usually lower which is a tough load for any tube amp imho. Keep that in mind. It sounds like an MTMWW.
Do measure the impedance/phase of your speakers if you can. A Dayton Audio DATSV3 is incredibly helpful if you can borrow one. Available at Parts Express. You can always return it if you don’t like it.
The ST35 as designed might be current starved in this setup. Then again, it doesn’t completely explain why you like the chinese design either.
Best,
Anand.
Last edited:
The red bass line labelled bass-shy/thin and the red mid line, at least the words harsh and thin plus the turqqoise line edgy and sharp. I doubt the amplifier actually measures per that chart, but the perception of the sound would be described that way verbally.
Yes, I am aware of the impedence of the 8 ohm drivers in parallel. Over the decades that I have been using these speakers, I have used a lot of amplifiers with no problems from one to the other. Interestingly, despite the 8 ohm drivers wired in parallel, the Dynaco ST-70 series ii always has sounded best on the 8 ohm output. The 4 ohm output on it sounds weak and lifeless, as does the 16 ohm output. Ditto on the cheap Chinese tube amplifier. I noted previously that the 4 ohm output on the ST-35 does eliminate the harshness in the midrange, but the performance is even worse in the sub-bass and totally unacceptable. So, I went back to the 8 ohm output on it as I have with the other two tube amps. A lot of people seem to use the 8 ohm output on tube amps despite having speakers that supposedly "should" be connected to the 4 ohm output.
One could test some other speakers that has a known impedance. The perceived problem might be in the
speaker<>amp interface, testing with other speakers could spread some light
Or one could connect a resistive power resistor to the amp and feed it with variable sine, noting the voltage
across the power resistor with a frequence independent voltmeter ( or an oscilloscope), this to figure out
the amp's frequency response both at low and high ( 15w) power.
speaker<>amp interface, testing with other speakers could spread some light
Or one could connect a resistive power resistor to the amp and feed it with variable sine, noting the voltage
across the power resistor with a frequence independent voltmeter ( or an oscilloscope), this to figure out
the amp's frequency response both at low and high ( 15w) power.
I have a sneaking suspicion that this situation has something to do with the add-on adjustable bias kit. If we can't find a logical alternate explanation on paper or in theory, I could unhook the bias add-on kit and try connecting the output tubes with the original single-resistor bias method instead. I'd have to obtain a good quality 100uF capacitor.
John,
Good. Now we’ve established you live in Jacksonville, FL; hence your request for scope enthusiasts in and around that area. I’m sure there is somebody local to you though. Ask around in the swap meet area of this forum.
Best,
Anand.
Good. Now we’ve established you live in Jacksonville, FL; hence your request for scope enthusiasts in and around that area. I’m sure there is somebody local to you though. Ask around in the swap meet area of this forum.
Best,
Anand.
I have both a VTA ST-70 and a Dynakit ST-35. Overall I prefer the sound of the
ST-35. I have found a few of my 6 ohm speakers just didn't play nice with the ST-35. One set can dip down to close to 2 ohms. Your description of how the ST-35 sounded is spot on with how this amp sounds with those speakers. I have JJ driver tubes and Sovtek EL-84M's in my ST-35.
So it could very well be the speakers. The same 6 ohm speakers connected to my ST-70 sound great. I have Russian 6P14P-P's in several other amps I've built and they sound great. I'm thinking of trying a set in the ST-35. I know the ER's are longer life, but the P's sound good and are cheaper. I've also read several reviews where people thought the 6P14P's and 6P14P-P's sounded better overall compared to the ER's, EB's , etc.
Have you tried any other speakers?
ST-35. I have found a few of my 6 ohm speakers just didn't play nice with the ST-35. One set can dip down to close to 2 ohms. Your description of how the ST-35 sounded is spot on with how this amp sounds with those speakers. I have JJ driver tubes and Sovtek EL-84M's in my ST-35.
So it could very well be the speakers. The same 6 ohm speakers connected to my ST-70 sound great. I have Russian 6P14P-P's in several other amps I've built and they sound great. I'm thinking of trying a set in the ST-35. I know the ER's are longer life, but the P's sound good and are cheaper. I've also read several reviews where people thought the 6P14P's and 6P14P-P's sounded better overall compared to the ER's, EB's , etc.
Have you tried any other speakers?
The only others I have are small surround sound type speakers with no bass to speak of, so that won't work. If it is a problem driving the speakers, it is the only amp to ever have a problem over several decades. It also seems like either the 8 ohm or 4 ohm output should sound good. Neither does.
Like this? I tested every single component when I got it from Mouser/DigiKey, then put it in the proper bag with the corresponding Dynakit parts, labelled the bag with the correct value, and then as I put each and every component into the circuit boards, I measured, matched L-R, and wrote down every single value on a worksheet:
Tonight I dropped it all into Excel to quadruple check all values and to see what my variances actually were. Of course, these numbers are only as precise as my inexpensive meter, but they should be close enough. Almost all resistors were 1% or less tolerance from Mouser/DigiKey to begin with. I can't find the sheet where I hand wrote the actual measured values for the bias control circuit board right now, but it's here somewhere. I can read all of the values on those few parts visually, and still have the labelled bags, and they all match spec with each one in the right holes. So everything is documented as the correct values, and we can rule that issue out.
@6A3sUMMER, you pointed me in a valuable direction. I am not convinced either. Referring to the measured voltages in post #172, why are my voltages universally low on pins #3 of the output tubes?
Using the bias adjustment directions, I measured 350mV at the 10 ohm resistors, as in post #173. Tonight, out of curiosity and frustration, I turned the four bias pots up as far as they would go, which gave me 366mV. Wow, what an improvement. I could hear the change as I sat on the floor between the speakers turning the pots. Most of the harsh midrange edge is gone now. The bass is significantly better, and the sub-bass is better (but still not enough).
Now the questions are:
- WHY?
- Which components do I change and to what values to get the potentiometers back to somewhere around center instead of maxed out, leaving me no more adjustability?
- Is 366mV actually the correct value in this case? Should it be slightly higher? I can't test that because the pots are maxed out.
And now, back to a little listening. Maybe I'll try the 6P14P-ER tubes again. Gee, I hope a circuit board doesn't snap in the process. The death grip of the sockets on the tubes is just ridiculous.
Last edited:
This is great news!
So you were underbiased all along? I’m more than a little surprised that the change from 35 mA to 36.6mA made that much of a difference. We should definitely be able to bias it higher, but just keep an eye on dissipation of the output tubes themselves. I don’t know what a safe operating point is for EL84 type tubes but I am sure others do. Of course, I would recommend trying different brands of EL84 tubes (matched ones) if available.
I don’t know much about the 12DW7, but trying different tubes is always an option in these circuits given that they are the amplification devices. It will definitely influence the sound.
I am fairly sure that these differences of underbiasing would be apparent on a distortion analyzer although a pair of calibrated ears is a great substitute as well!
Regarding measuring parts, you do what works for you. I usually just measure immediately before installing the component and have the schematic in front of me. And I use a calibrated bench meter for a lot of the parts instead of a DMM. But both are fine choices for tube designs which are not that persnickety for matching of parts to be honest (except phonostages). I have too many projects to systematically arrange them in small bags and categorize them on Excel as you have done. I just pick them out of the mouser bag, start measuring, match with the schematic and immediately solder them. Kudos to your diligence and fortitude!
Best,
Anand.
So you were underbiased all along? I’m more than a little surprised that the change from 35 mA to 36.6mA made that much of a difference. We should definitely be able to bias it higher, but just keep an eye on dissipation of the output tubes themselves. I don’t know what a safe operating point is for EL84 type tubes but I am sure others do. Of course, I would recommend trying different brands of EL84 tubes (matched ones) if available.
I don’t know much about the 12DW7, but trying different tubes is always an option in these circuits given that they are the amplification devices. It will definitely influence the sound.
I am fairly sure that these differences of underbiasing would be apparent on a distortion analyzer although a pair of calibrated ears is a great substitute as well!
Regarding measuring parts, you do what works for you. I usually just measure immediately before installing the component and have the schematic in front of me. And I use a calibrated bench meter for a lot of the parts instead of a DMM. But both are fine choices for tube designs which are not that persnickety for matching of parts to be honest (except phonostages). I have too many projects to systematically arrange them in small bags and categorize them on Excel as you have done. I just pick them out of the mouser bag, start measuring, match with the schematic and immediately solder them. Kudos to your diligence and fortitude!
Best,
Anand.
Last edited:
NGO - Stephe built monoblock SPPs using EL84's and compared biasing them with 270R vs 330R resistors, and found the latter cooler bias produced a significantly "brighter" frequency response. See particularly, her video, here at the 6m22s mark. By the way, Stephe has a great video playlist and she is a great resource for the less experienced.
Now there's probably a lot of differences between SPPs and ST35s but from your schematic, I think the lowest bias resistance you can set on yours may be: 470R parallel with 1,000R plus zero R on the pot plus another 10R, which is 330R. If you changed out the four 470R 1/2 watt cathode resistors in favour of say 360R, your minimum bias resistance seems to come down to 275R. Or 330Rs would give you a 258R minimum. You may want to check your dissipation carefully, though. Stephe acknowledges that she is running the EL84s hot.
Have fun
Simon
Now there's probably a lot of differences between SPPs and ST35s but from your schematic, I think the lowest bias resistance you can set on yours may be: 470R parallel with 1,000R plus zero R on the pot plus another 10R, which is 330R. If you changed out the four 470R 1/2 watt cathode resistors in favour of say 360R, your minimum bias resistance seems to come down to 275R. Or 330Rs would give you a 258R minimum. You may want to check your dissipation carefully, though. Stephe acknowledges that she is running the EL84s hot.
Have fun
Simon
I have watched all of her videos.
It's discussed somewhere here in this thread or maybe the one devoted to possible power supply mods for this amp. IFFFF the current is 35mA, which I am starting to doubt even though that is what it's supposed to be when measuring 350mV at the 10 ohm resistor, then the dissipation is at or slightly above 100%. However, the voltage being ~12.4V instead of the 13.5V (see post #172) as specified on all four pins #3 is causing that measurement to be inaccurrate, (correct?), and that is the source of my problem here, correct? One of you Ohm's Law experts help me out here with the voltage drop across the bias resistors given the voltage deviance from spec please.
Encouraged by the drastic increase in sound quality as I increased the bias last night, and not wanting to burn up the 6P14P tubes, I put the extremely durable 6P14P-ER tubes back in. I started at 365mV. Nope. Not good sound. However, with those tubes, I had a lot of extra room to turn up the pots. I tried 370mV. Nope. Not good sound. I tried 380mV. Nope. Not good sound. So I kept going to 390mV and finally things started to improve. So I went to 400mV. Now we are sounding pretty good. Unfortunately, the pots are now maxed out at 400mV, so I have zero flexibility. The directions state,
The current setting of each EL84 output tube should now be 35 mA. This can be adjusted to your preference via the on-board bias potentiometers, providing a useable range of 320 to 450 mV (32 to 45 mA).
Nope. I'm only getting a range of about 300 to 365 with the 6P14P and 300 to 400 with the 6P14P-ER.
Something odd is going on here and I suspect it may be related to the low voltage on pins #3. What do I need to measure to find out what's going on here so I can get this fixed? The spec for bias is 350mV = 35mA. It sounds awful. I can't get good sound out of either quad of tubes without maxing out the pots when they should, ideally, be somewhere in the middle. Maxed out means 400mV at the test points for the 6P14P-ER and 366mV on the plain 6P14P. The full schematic, including the bias circuitry, is in post #1.
The 6P14P tubes now look like they have been smoking two packs of cigarettes a day. It's normal for them to spew some stuff inside the tube, but it was minimal when they were in the little Chinese amplifier for a few months. If I recall correctly, it runs them at about 90% of max dissipation.
Another thing that I found out last night is that it takes a full 2 hours of warm up time to get the amplifier cooking (literally) and for the bias to settle at its final value. Once I got the amp fully heated up with the 6P14P-ER tubes maxed out at 400mV, I actually enjoyed the sound of the amplifier for the first time. At least that is a step in the right direction. Hopefully someone here can tell me what the next steps should be.
Given the above new details, do you think this is the ultimate solution, and if so, which value?
Last edited:
Pin #3 (cathode) voltage depends on the tubes, and you are within a volt or so from the Dyna specs. Why do you suspect those? I guess the problem is elsewhere.
Looking at your measured voltages I noticed you measured 10-15 mV on the output tube grid pins. That should be 0 according to specs. Are you measuring DC voltages? That makes me wonder if your Multimeter is accurately measuring dc volts, especially in the mV range. Perhaps you could try to compare to a friend’s to ascertain accuracy. The fact that you are hitting stops on your bias range seems to negate the theory that only the meter is at fault.
Looking at your measured voltages I noticed you measured 10-15 mV on the output tube grid pins. That should be 0 according to specs. Are you measuring DC voltages? That makes me wonder if your Multimeter is accurately measuring dc volts, especially in the mV range. Perhaps you could try to compare to a friend’s to ascertain accuracy. The fact that you are hitting stops on your bias range seems to negate the theory that only the meter is at fault.
Approximately 374V.
Ah, thanks. I only tested with the 6P14P, and only then with them at 35mA. Perhaps I will retest with 6P14P-ER next time I remove the amplifier.
Yes.
I have no reason to supect it, but anything is possible. I wonder if I have something with a known mV source around to test. I don't have access to another meter unless I buy another one.
I did notice that. I have no idea what would cause that unless it's the tubes themselves, or the meter somehow measuring something that is not there. All of the component values in the whole amplifier have been confirmed 4 times and actually tested 2 times. I was extremely careful with all of the solder joints and therefore doubt there is a bad one anywhere.
I only tested with the 6P14P tubes, and only then with them at 35mA. Perhaps I will retest with 6P14P-ER next time I remove the amplifier.
Something ain't right here, and I can't figure out what it is. It sounds good with the bias pots maxed out, but it shouldn't require that. Regardless of possible meter issues measuring millivolts, the fact is that the amp only sounds good with the bias pots maxed out, for both the 6P14P and the 6P14P-ER.
I don't want to change the design. It should operate and sound good with the given design, and it does with the bias pots maxed out.
Last edited:
One could also get a quad of matched EL84 which is what the transformers are made for.
Reducing number of unknown will enhance trouble shooting.
Reducing number of unknown will enhance trouble shooting.