88% is indeed conservative for class A operation. But something still does not look quite right, did you measure the OPT? What is the primary DC resistance? And the impedance? It should be 3.5k~5k.
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OPT measurements (no tube):
Full primary DC resistance (pin3 to filter cap) = 370 ohm, on both OPTs.
Pin4 to filter cap (UL tap) = 145 ohm.
35H primary inductance.
Impedance:
@8ohm, 1vAC secondary => 23vAC Primary, 9.8v UL tap.
@4ohm, 1vAC secondary => 38vAC Primary, 16.5 UL tap.
UL tap ratio is 40%
8 ohm = (23^2*8) 4200 ohm reflected.
4 ohm = (38^2*4) 5800 ohm reflected.
Everything seems to match other Single-Ended OPTs from a quick google search.
Full primary DC resistance (pin3 to filter cap) = 370 ohm, on both OPTs.
Pin4 to filter cap (UL tap) = 145 ohm.
35H primary inductance.
Impedance:
@8ohm, 1vAC secondary => 23vAC Primary, 9.8v UL tap.
@4ohm, 1vAC secondary => 38vAC Primary, 16.5 UL tap.
UL tap ratio is 40%
8 ohm = (23^2*8) 4200 ohm reflected.
4 ohm = (38^2*4) 5800 ohm reflected.
Everything seems to match other Single-Ended OPTs from a quick google search.
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Great, it looks like everything is in order except for the bias - bear in mind that for UL, both the plate and screen are delivering power, so the Pda could be higher (25+8=33W). At 18W, that's a bit conservative.
Thanks jazbo8.
The volume this amp provides is above my comfortable listening level anyway.
I have already asked this (DF96 kindly replied), but I want your opinion too.
Many people suggest that a screen resistor is essential to tube life (against screen arcing),
but THIS document at Figure 3A suggest that screen resistor can be skipped in a UL and cathode biased amps.
What is your thought on this?
The volume this amp provides is above my comfortable listening level anyway.
I have already asked this (DF96 kindly replied), but I want your opinion too.
Many people suggest that a screen resistor is essential to tube life (against screen arcing),
but THIS document at Figure 3A suggest that screen resistor can be skipped in a UL and cathode biased amps.
What is your thought on this?
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Yes it could be skipped if the amp is used mainly for moderate playback levels, but adding a small resistor as DF96 suggested should give you a peace of mind even when you crank it up during parties, and it does not impact the performance at low levels anyway.
Installed 100ohm screen grid resistors as suggested.
Installed 6L6GC, they sound a lot better, cleaner, beefier, and they glow with beautiful blue aura inside.
Maybe because of this:
I just realized that Pin-1 of the power tube is not connected thus effectively leaving the EL34 as a Tetrode (huge kink). 🙄
This amp was designed for 6L6 (Chinese 6p3p) where pin8 and pin1 connected internally.
If I solder pin-1 to ground for EL34, this would bypass the cathode biasing resistor with 6L6 which will destroy the tube immediately.
Should I jumper pin1 to pin8 to enable the Suppressor grid for EL34?
I don't think a Tetrode is good for HiFi...
Installed 6L6GC, they sound a lot better, cleaner, beefier, and they glow with beautiful blue aura inside.
Maybe because of this:
I just realized that Pin-1 of the power tube is not connected thus effectively leaving the EL34 as a Tetrode (huge kink). 🙄
This amp was designed for 6L6 (Chinese 6p3p) where pin8 and pin1 connected internally.
If I solder pin-1 to ground for EL34, this would bypass the cathode biasing resistor with 6L6 which will destroy the tube immediately.
Should I jumper pin1 to pin8 to enable the Suppressor grid for EL34?
I don't think a Tetrode is good for HiFi...
Yes, you should do it. Did you play with the cathode resistor to hear how the bias change the sound?
Not yet.
The kink of the Tetrode wired EL34 generated more distortion than any possible freezing bias of a Pentode.
That may explain the high distortion % measurement of some people with these amps.
Learning about Tetrodes, I read that stray electrons that backlash out of the plate because of the force they hit it (secondary emission),
are attracted back to the screen grid which is also positive and generating unnecessary current, especially if you don't have a screen grid resistor.
By connecting the Suppressor grid to ground these secondary emission electrons are collected by this grid thus preventing screen grid current.
Furthermore, Tetrodes tend to oscillate when used for amplification. Overall, a total disaster in amplifiers.
Anyway, all the people who got their Chinese Single-Ended amplifiers with EL34 wired as Tetrode (floating/unconnected pin-1), should not wonder why the amp distorts like crazy at high volumes.
I would like to add these videos to this thread:
Guitar Amp Vacuum Tubes Part 1: Triodes and Tetrodes.
https://www.youtube.com/watch?v=Xo4nVjj4NSE
Guitar Amp Tubes Part 2: Pentodes and Beam Tetrodes
https://www.youtube.com/watch?v=Zh_51RP4V6I
Guitar Amp Vacuum Tubes Part 1: Triodes and Tetrodes.
https://www.youtube.com/watch?v=Xo4nVjj4NSE
Guitar Amp Tubes Part 2: Pentodes and Beam Tetrodes
https://www.youtube.com/watch?v=Zh_51RP4V6I
No. The suppressor grid sends the secondary electrons back to the anode. It (g3) repels them, not collects them.James Freeman said:
Tetrodes were often used for high power amplifiers, for audio and RF. They need good screen grid supply arrangements (unlikely in a cheap audio amp) as under some conditions the screen grid current can be negative. Given this, they can be quite linear.
Oops, correct.
After replacing to 6L6GC's the power transformer was way cooler than with EL34's.
Probably because the filament current dropped from 3A to 1.8A.
"unlikely in a cheap audio amp"
And that means the majority of them.
Or did you see an exceptional one with Tetrodes?
With EL34s.
Before soldering G3 to Cathode (pin 1 to 8) I'll measure the voltage across the screen resistor (100 ohm) to know the screen current in Tetrode.
After soldering 1 to 8 I'll measure the screen current again in Pentode.
This should be interesting...
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Before and after connecting the Suppressor grid the current of the Screen grid remained the same.
I guess not many secondary emission electrons bounced from the anode to the screen, so the screen current did not change between Tetrode and Pentode.
Screen current at idle: 300mV over 100R so 3mA.
Screen Power dissipation: 0.003A * 356V = 1W.
In total, I did two mods for now.
1. 100 ohm screen resistor.
2. Bridging pin1 and pin8 on the socket.
If it were a push-pull amp I would notice that something wrong immediately as the resulting sound would have been massive crossover distortion (buzzing) even at very low volumes because the kink in the Tetrode.
Since it is a single ended amp, I guess the power tubes did not swing enough for me to hear a distortion (did not each the kinks), it would have sounded like a strong version of asymmetric distortion.
PS, I test with a 1kHz sinewave at full volume of the amp, very loud but not damaging the speakers or my hearing.
The amp biased cold (20W per tube) and it does not put much power at the output (around 8W) so I don't worry about cranking it.
Cheers
I guess not many secondary emission electrons bounced from the anode to the screen, so the screen current did not change between Tetrode and Pentode.
Screen current at idle: 300mV over 100R so 3mA.
Screen Power dissipation: 0.003A * 356V = 1W.
In total, I did two mods for now.
1. 100 ohm screen resistor.
2. Bridging pin1 and pin8 on the socket.
If it were a push-pull amp I would notice that something wrong immediately as the resulting sound would have been massive crossover distortion (buzzing) even at very low volumes because the kink in the Tetrode.
Since it is a single ended amp, I guess the power tubes did not swing enough for me to hear a distortion (did not each the kinks), it would have sounded like a strong version of asymmetric distortion.
PS, I test with a 1kHz sinewave at full volume of the amp, very loud but not damaging the speakers or my hearing.
The amp biased cold (20W per tube) and it does not put much power at the output (around 8W) so I don't worry about cranking it.
Cheers
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For the amps that use EL34B, the "supressor" is internally connected in the tube (it's a beam power tube). If you run a SPICE model on the amps, the distortion is high at about any practical power, el34, 6ca7, kt77, 6l6, 5% distortion hits well underr the claimed power levels of 11W.
Thanks, now that you say, I took a closer look.
Actually it looks like a pre-wired Pentode, not a Beam Tetrode which lacks a real suppressor grid.
No wonder the measurement had no change.
With the tubes I received Pin1 leads to nothing inside the tube, just unsoldered wire in vacuum.
There is a small Cathode to Suppressor jumper wire right under the bottom mica disc.
I can clearly see the suppressor grid through the holes in the Plate, which I can clearly see their posts which the suppressor grid is wound around from the bottom, which I can clearly see jumpered to the Cathode.
In fact, in this Psvane EL34-B tube Pin8 soldered directly to the Suppressor grid which then jumpered to the cathode.
Definitely a Pentode but a pre-wired one.
A really stupid thing to do because not all amps connect g3 to the cathode.
BTW,
I took some measurements around the Triode and plotted a load line.
The result shows that a 2v -+ at the triode grid will swing the output 25v +- which is exactly the negative value of the grid of the power tubes.
No unnecessary clipping, no extra gain, everything in the linear range, with maxed volume (input) pot.
The ac swings quite symmetrically in the triode, and the gain is very low (25/2 = 15), just enough to swing the power grids to maximum with professional line level (1.736vac peak) at the triode input, and not a volt more.
By design?
Actually it looks like a pre-wired Pentode, not a Beam Tetrode which lacks a real suppressor grid.
No wonder the measurement had no change.
With the tubes I received Pin1 leads to nothing inside the tube, just unsoldered wire in vacuum.
There is a small Cathode to Suppressor jumper wire right under the bottom mica disc.
I can clearly see the suppressor grid through the holes in the Plate, which I can clearly see their posts which the suppressor grid is wound around from the bottom, which I can clearly see jumpered to the Cathode.
In fact, in this Psvane EL34-B tube Pin8 soldered directly to the Suppressor grid which then jumpered to the cathode.
Definitely a Pentode but a pre-wired one.
A really stupid thing to do because not all amps connect g3 to the cathode.
BTW,
I took some measurements around the Triode and plotted a load line.
The result shows that a 2v -+ at the triode grid will swing the output 25v +- which is exactly the negative value of the grid of the power tubes.
No unnecessary clipping, no extra gain, everything in the linear range, with maxed volume (input) pot.
The ac swings quite symmetrically in the triode, and the gain is very low (25/2 = 15), just enough to swing the power grids to maximum with professional line level (1.736vac peak) at the triode input, and not a volt more.
By design?
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Almost all pentode applications wire g3 to the cathode. There are some pentodes with separate connections for those few applications where this is needed.
Back to the question of why the measured DC after the rectifier is a lot less than the perfect 1.414 ratio of measured AC.
My DVM reads 300vac before SS rectifier and 380vDC after rectifier at the cap when the tubes are warmed up.
This is 50vDC short of the 1.414 value with SSR (with power tubes).
When the tubes are cold (at switch on) the DC reading starts at 430vDC and slowly drops down to 380 (or less) when the tubes are warm.
When removing the output tubes the voltage value stays at 430vDC.
This can only mean that the power tube does not consume current from the power supply until its filament is hot.
I understand that the power supply sags, but why does the AC reading stays the same??
It's 300v AC before rectifier, when DC after rectifier is 430v or 380v or 350v.
Maybe the AC waveform is so distorted at load that my DVM reads it wrong?
If I read the Choke resistance and voltage drop, could I say that the resulting current by ohms law is the total current consumption from the power supply?
Shouldn't changing the power tubes with lower filament current leave the power supply more flux before core saturation thus less sag on the AC HT+?
As I understand, all the secondaries are wound on the same core so core saturation depends on the current from all secondaries including the filaments.
I want to get down to the bottom of this, of why does my DC reading is far less from the 1.414 ratio of the measured AC.
I should mention that the power supply has a typical "Pi filter" (Cap, Choke, Cap) after the rectifier.
I got a signal generator and DVM , but no Scope.
Should I start by measuring the PWT secondary resistance, and choke resistance and voltage drop?
Help and knowledge would be appreciated.
My DVM reads 300vac before SS rectifier and 380vDC after rectifier at the cap when the tubes are warmed up.
This is 50vDC short of the 1.414 value with SSR (with power tubes).
When the tubes are cold (at switch on) the DC reading starts at 430vDC and slowly drops down to 380 (or less) when the tubes are warm.
When removing the output tubes the voltage value stays at 430vDC.
This can only mean that the power tube does not consume current from the power supply until its filament is hot.
I understand that the power supply sags, but why does the AC reading stays the same??
It's 300v AC before rectifier, when DC after rectifier is 430v or 380v or 350v.
Maybe the AC waveform is so distorted at load that my DVM reads it wrong?
If I read the Choke resistance and voltage drop, could I say that the resulting current by ohms law is the total current consumption from the power supply?
Shouldn't changing the power tubes with lower filament current leave the power supply more flux before core saturation thus less sag on the AC HT+?
As I understand, all the secondaries are wound on the same core so core saturation depends on the current from all secondaries including the filaments.
I want to get down to the bottom of this, of why does my DC reading is far less from the 1.414 ratio of the measured AC.
I should mention that the power supply has a typical "Pi filter" (Cap, Choke, Cap) after the rectifier.
I got a signal generator and DVM , but no Scope.
Should I start by measuring the PWT secondary resistance, and choke resistance and voltage drop?
Help and knowledge would be appreciated.
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Yes.James Freeman said:
You should get some AC sag, but most of the sag will be across the rectifier not the secondary. Also, the secondary sag will only be near the peak so may have less effect than you expect - but it will depend on exactly how your meter works.
For a smoothing choke, yes - the drop across the choke will be mainly DC.
Saturation is not the issue. Voltage drop in the windings due to their resistance is the issue. Less heater current means less voltage drop in the primary winding resistance so slightly higher AC for the other secondary windings.
Yes. The more secondary current you draw the further the transformer moves away from core saturation.
To save me searching for it, exactly what is your PSU circuit?
Instead of the two 220ohm resistors in parallel there is a choke.
Instead 1.2K to triodes there is a 1.5K.
No bleed resistor.
SSR instead of Tube (I replaced it).
5V instead of 6.3+resistor to rectifier.
All caps rated 450V.
300Vx2 AC.
Voltage measurement on tubes: POST #80
A typical Pi PSU.
Note that ALL the current passes through the Choke, should be around 150mA.
I'll do the resistance measurements and post.
The amp is upside down and open, just ask me for any measurement.
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With a solid-state rectifier there are really only two sources of PSU droop under load:
1. winding resistance (both secondary and reflected primary)
2. reservoir capacitor ripple - the DC value out is approximately 'peak AC' - 0.5*'ripple pk-pk'
Is it possible that the reservoir cap is much smaller than 450uF or disconnected? 450uF would be far too much for a 5Z4 to feed, and quite large for even a silicon rectifier.
1. winding resistance (both secondary and reflected primary)
2. reservoir capacitor ripple - the DC value out is approximately 'peak AC' - 0.5*'ripple pk-pk'
Is it possible that the reservoir cap is much smaller than 450uF or disconnected? 450uF would be far too much for a 5Z4 to feed, and quite large for even a silicon rectifier.
Choke resistance is 50 Ohm.
Whole Secondary resistance is 115 ohm, Half (tap to ground) 57 ohm.
Halves are in series.
Voltage before Choke (on first cap after rectifier) 375v
Voltage after Choke (on second cap) 368.
(375-368)/50 = 0.140 = 140mA
The amp draws 140mA± from the power supply.
Each power tubes draws 63mA± (24.5/390), plus triodes.
Voltage drop on the secondary is 0.14*115= 16V ??
Whole Secondary resistance is 115 ohm, Half (tap to ground) 57 ohm.
Halves are in series.
Voltage before Choke (on first cap after rectifier) 375v
Voltage after Choke (on second cap) 368.
(375-368)/50 = 0.140 = 140mA
The amp draws 140mA± from the power supply.
Each power tubes draws 63mA± (24.5/390), plus triodes.
Voltage drop on the secondary is 0.14*115= 16V ??
How so?
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