I bought a Chinese, Single-Ended, Class A, EL34, Hi-Fi stereo tube amplifier.
I have few questions for you about Triode/Pentode/Ultra-Linear, NFB and Cathode Biasing.
Schematic does not show output transformer taps, or power transformer filament taps.
The amp came with no 100 Ohm screen resistor which was added by me later.
I also bridged Pin1 to Pin8 on the power tube socket, because Pin1 was not connected (not good for true Pentodes).
I replaced the Tube Rectifier with Solid State made of two diodes.
VU meters are powered from the 6.3vAC filament tap, and receive input from the 8 ohm Speaker tap.
After a quick goolge search I understand that they are all more or less the same.
Note: I've been modding and tweaking my guitar tube amp for years, so don't worry about electrocution.
Mode Switch:
I want to install a UL/Pentode/Triode switch using two DPDTs.
1st DPDT, center to Screen Grid through a 100R 3W metal film resistor, UP to UL tap, Down to 2nd center DPDT.
2nd DPDT, UP to Plate (triode), Down to HT (pentode).
Is this correct or should I add something?
Why the small 100R resistor, guitar amps have 1K there?
*The switches are of the sturdy 110v 20A type.
NFB:
First I want to try completely remove the 10K NFB resistor to get a "taste" of the tone of each mode without the NFB damping its response.
In fact, I don't want NFB at all because my Solid State amp does just that; I want to hear distinct changes between tubes and modes interacting with my speakers impedance. I want the amp to be unpredictable, alive and distort at the impedance peaks.
So no NFB for me, at first with this Hi-Fi.
My guitar tube amp (push-pull 6L6GC) has NFB pot and switch4 so I can tweak it to taste of completely disconnect the loop.
No NFB just screams, growls, and has ballz, I like no NFB for guitar.
What do you fellows suggest for Hi-Fi, NFB or not? Keep in mind that I have my SS for flat boring sound of 0% THD, endless Damping factor; I want Character and Ballz from tubes.
Cathode Biasing:
This is my first experience with Single-Ended self biasing Resistor tube amp.
Why is it a "one size fits all" this cathode biasing resistor (EL34,6L6GC, 6p3p)?
What current should I measure over this cathode resistor?
How does the B+ plate voltage relates to the biasing of single-ended?
Why some have 220R and others have up to 680R for this cathode resistor?
I appreciate the help.
I have few questions for you about Triode/Pentode/Ultra-Linear, NFB and Cathode Biasing.
Schematic does not show output transformer taps, or power transformer filament taps.
The amp came with no 100 Ohm screen resistor which was added by me later.
I also bridged Pin1 to Pin8 on the power tube socket, because Pin1 was not connected (not good for true Pentodes).
I replaced the Tube Rectifier with Solid State made of two diodes.
VU meters are powered from the 6.3vAC filament tap, and receive input from the 8 ohm Speaker tap.
After a quick goolge search I understand that they are all more or less the same.
Note: I've been modding and tweaking my guitar tube amp for years, so don't worry about electrocution.
Mode Switch:
I want to install a UL/Pentode/Triode switch using two DPDTs.
1st DPDT, center to Screen Grid through a 100R 3W metal film resistor, UP to UL tap, Down to 2nd center DPDT.
2nd DPDT, UP to Plate (triode), Down to HT (pentode).
Is this correct or should I add something?
Why the small 100R resistor, guitar amps have 1K there?
*The switches are of the sturdy 110v 20A type.
NFB:
First I want to try completely remove the 10K NFB resistor to get a "taste" of the tone of each mode without the NFB damping its response.
In fact, I don't want NFB at all because my Solid State amp does just that; I want to hear distinct changes between tubes and modes interacting with my speakers impedance. I want the amp to be unpredictable, alive and distort at the impedance peaks.
So no NFB for me, at first with this Hi-Fi.
My guitar tube amp (push-pull 6L6GC) has NFB pot and switch4 so I can tweak it to taste of completely disconnect the loop.
No NFB just screams, growls, and has ballz, I like no NFB for guitar.
What do you fellows suggest for Hi-Fi, NFB or not? Keep in mind that I have my SS for flat boring sound of 0% THD, endless Damping factor; I want Character and Ballz from tubes.
Cathode Biasing:
This is my first experience with Single-Ended self biasing Resistor tube amp.
Why is it a "one size fits all" this cathode biasing resistor (EL34,6L6GC, 6p3p)?
What current should I measure over this cathode resistor?
How does the B+ plate voltage relates to the biasing of single-ended?
Why some have 220R and others have up to 680R for this cathode resistor?
I appreciate the help.
Last edited:
Bumping as i wish to learn something as well.
Anyway, i assume you have new OPT pairs having UL taps?
Your schematic shows OPT possible for Pentode or Triode only.
edit: now your post shows another schematic with UL capable OPT
For safety measures, i would install a resistor across the 8ohm secondary to ground, say, 56R to 100R at 5Watt. This avoids damaging your OPT in case you accidentally disconnect your speaker on Pentode Mode with music playing.
Anyway, i assume you have new OPT pairs having UL taps?
Your schematic shows OPT possible for Pentode or Triode only.
edit: now your post shows another schematic with UL capable OPT
For safety measures, i would install a resistor across the 8ohm secondary to ground, say, 56R to 100R at 5Watt. This avoids damaging your OPT in case you accidentally disconnect your speaker on Pentode Mode with music playing.
Most likely guys here would answer something along the lines of "try and listen to both and keep the one you like".
Last edited:
The OPT has UL tap and it's the default wiring mode this amp comes with.
Yes, a short or a resistor is good protection against flyback voltage that may short the OPT windings.
Do you think the OPT is that flimsy that is may immediately short when disconnecting the load while playing music?
In push-pull AB guitar amp the transformer is safe when no signal goes through it without any load (open circuit).
What about single ended? There should be no flyback voltage when there is no AC signal through it as well.
Yes, a short or a resistor is good protection against flyback voltage that may short the OPT windings.
Do you think the OPT is that flimsy that is may immediately short when disconnecting the load while playing music?
In push-pull AB guitar amp the transformer is safe when no signal goes through it without any load (open circuit).
What about single ended? There should be no flyback voltage when there is no AC signal through it as well.
Let's wait response from more experienced member, shall we?
However, i am inclined to think that regardless of the class, pushpull or single ended, the OPT is in danger of shorting with disconnected load on Pentode mode.
Regardless of the class (bias point), I imagine the loadline would be horizontal (or close to horizontal) on the plate characteristic and a small change in Vgk would shoot Vak way up high that would endanger the winding insulation from high voltage breakdown.
No signal is what keeps the transformer safe, or at least that's what i believe.
If you mean the 100R in the first stage cathode circuit, that is there to provide a point to insert negative feedback. Too large a value would upset the first stage bias, and would increase the output impedance of the first stage by too much.James Freeman said:
You can have Hi-Fi or "Character and Ballz" - you need to choose which you prefer. To get Hi-Fi NFB is a great help. If you find Hi-Fi to be "flat boring sound " then don't pursue Hi-Fi.
You should assume that all OPT are "that flimsy". The fact that you may have got away with it in the past is no guarantee that this will continue (even NASA discovered that!). It may be that SE amps are more prone to this than P-P.
No, I mean Screen Grid Resistor.
The schematics don't show it, but I read that when in Triode mode this resistor is mandatory.
Thanks.
So, practically everybody is doing this on diyaudio with tube amps?
What about the biasing cathode resistor value then?
With Push-Pull Class AB, I bias the current across the 1ohm cathode resistor to 70% (or lower) of tube power rating (0.7 * W/PlateV).
Should I bias at 100% plate dissipation of the tube in Class A? Current across cathode resistor = [W/(plateV-cathodeV)].
Although because the resistor is fixed, I can only check what plate dissipation the build in resistor creates.
Last edited:
Sorry, my Crystal Ball is away for repair so I couldn't see the other 100R which wasn't there in the circuit diagram but is apparently there in the amp in front of you somewhere in the world.
For SE Class A you probably need to bias somewhere near maximum anode dissipation. The cathode resistor value will depend on valve type and supply rail voltage.
The usual advice is never ever run a tube amp without an output load. If you think you are likely to accidentally do this then you need, as a minimum, a CR (Zobel) network across the OPT secondary. If you plan to do this regularly then you may need a resistor which will absorb a significant amount of output power.
For SE Class A you probably need to bias somewhere near maximum anode dissipation. The cathode resistor value will depend on valve type and supply rail voltage.
The usual advice is never ever run a tube amp without an output load. If you think you are likely to accidentally do this then you need, as a minimum, a CR (Zobel) network across the OPT secondary. If you plan to do this regularly then you may need a resistor which will absorb a significant amount of output power.
If I want to use only one side, can I pull the preamp and power tubes from the sockets without harming the transformer?
By the look of it, DC should do no harm to the OPT.
I intend to use a tube socket solid state rectifier instead of the tube one.
What benefits will there be?
"Screen Grid through a 100R 3W metal film resistor,"
So why the 100ohm g2 resistor at all? Why not connecting straight to the plate like other triode schematics?
By the look of it, DC should do no harm to the OPT.
I intend to use a tube socket solid state rectifier instead of the tube one.
What benefits will there be?
It looks like I clearly state Screen Grid resistor in the Mode Switching (Triode/Pentode/UL) section, so it can NOT be a preamp cathode resistor.DF96 said:
"Screen Grid through a 100R 3W metal film resistor,"
So why the 100ohm g2 resistor at all? Why not connecting straight to the plate like other triode schematics?
Last edited:
My apologies - I missed that part. A 100R resistor here is acting as a stopper, to prevent RF parasitic oscillation. The exact value is not too critical. A much larger value may be intended to reduce screen grid dissipation when the anode voltage drops too low, but I am not sure. Bear in mind that guitar amps are not designed to reproduce the input signal.James Freeman said:
Removing the valves for one channel should do no harm, provided that the PSU caps can cope with any increase in voltage due to reduced PSU load. Some Chinese designs sail close to the wind on this. Similarly, replacing the vacuum rectifier with solid-state will increase voltage so check the caps. If the unit has been used for some time as it is then the caps may have become used to the voltage they are at, so may need reforming to a higher voltage again.
Absolutely!
Seen it happen. A 450V rated cap used for some time on 325V actually exploded when used at 430 for a while,--It wasn't a very old or a crap make cap either.....
As to a Pentode SE without feedback--To my ear--They sound Horrible, Coarse, Harsh and rough!
Even P-P Pentode sounds absolutely Horrible with No NFB.
Triode-mode/Real Triode however--is Completely Different, and can be run no NFB all things being equal.
NFB ISNT the boogey-man everyone thinks it is. It depends on How its used--and more importantly How MUCH!
Well implemented NFB will stabilise the amp, increase its bandwidth, reduce O/P impedance and lower distortion, --without imparting a dead lifeless sound.
Poorly implemented, and NFB can make a good amp bad, or a poor amp unusable!
Ive recently messed around with a Cheapo-Chinese P-P amp. The OPT's aint that great--(Although very large for EL84) and really HAVE to have a Conjunctive-filter across the Primary. I settled on 1000pF and 2.2K to rid the ringing on square-wave and prevent radio transmitter operation,--It oscillated that bad without NFB during my testing and modifications!
Last edited:
A healthy electrolytic when in use has just enough leakage current to perform just enough electrolysis to produce just enough new oxide to offset oxide layer depletion so as to maintain that leakage current. Drop the voltage for a long period and the oxide layer can thin a little because at first the leakage with the lower voltage is much smaller so doesn't replace enough oxide. The opposite happens with careful reforming. If you suddenly apply too large a voltage then the large leakage current causes heating and gas build up so the thing goes BANG!
Hmmm...
So for a healthy HT+ 450VDC (more like 300v for this amp), would you suggest a 630V cap or a typical 500v would do?
I read that 20% above working voltage as a rule among electrolytic caps.
*I am going to use stock for a while anyway.
As for NFB,
I'm going to play with some EL34 and 6L6GC from my guitar amp and try all output modes without NFB at first for maximum speaker impedance interaction.
I quite like the raw grinding Pentode without NFB sound in my guitar amp.
I got Polk RTi A3 speakers with a typical impedance for speakers with crossover.
So I expect some punch in the 30Hz, 80Hz and 2kHz regions, which should quite match what the human ear likes.
As I understand with no NFB, Pentode mode has the lowest damping factor and biggest interaction with speaker impedance.
Ultra-Linear has less interaction and more damping factor, and Triode has the biggest damping factor and almost doesn't reflect the speakers impedance (sort of like SS).
But, Triode has the nicest distortion characteristics right?
So for a healthy HT+ 450VDC (more like 300v for this amp), would you suggest a 630V cap or a typical 500v would do?
I read that 20% above working voltage as a rule among electrolytic caps.
*I am going to use stock for a while anyway.
As for NFB,
I'm going to play with some EL34 and 6L6GC from my guitar amp and try all output modes without NFB at first for maximum speaker impedance interaction.
I quite like the raw grinding Pentode without NFB sound in my guitar amp.
I got Polk RTi A3 speakers with a typical impedance for speakers with crossover.
An externally hosted image should be here but it was not working when we last tested it.
So I expect some punch in the 30Hz, 80Hz and 2kHz regions, which should quite match what the human ear likes.
As I understand with no NFB, Pentode mode has the lowest damping factor and biggest interaction with speaker impedance.
Ultra-Linear has less interaction and more damping factor, and Triode has the biggest damping factor and almost doesn't reflect the speakers impedance (sort of like SS).
But, Triode has the nicest distortion characteristics right?
Last edited:
So much confusion.......
You are getting advice for a stereo playback system but you want advice for a guitar amp, this thread probably get moved to instruments and amps to cut down on confusion.
For guitar application a lot of people like the pentode distortion characteristics. Removing the NFB loop, this is probably necessary anyway to increase gain to be used for guitar. Also the higher output Z is most favorable with guitar speakers. My advice, place a filter somewhere in the amp to scoop the mids if using passive guitar pickups, the input signal won't be flat, there is a bump in the mid range that you'll want to even out. Or if you end up with more then enough gain (which I doubt) you can add a whole tone stack, to do this though you might have to add another stage.
Also the advice on the capacitors is that you don't have to buy uprated capacitors unless the voltage rating will be exceeded. DF96 explained it very clearly. IF you are below the current rating, like using 450v caps and the B+ is only 300, after removing the tubes (load) from one channel and adding a SS rectifier the voltage might jump up to 400V. This is still under the rating but to be safe and not have them blow up you will want to "reform" them. I use high value resistors and step them down to limit the current.
You are getting advice for a stereo playback system but you want advice for a guitar amp, this thread probably get moved to instruments and amps to cut down on confusion.
For guitar application a lot of people like the pentode distortion characteristics. Removing the NFB loop, this is probably necessary anyway to increase gain to be used for guitar. Also the higher output Z is most favorable with guitar speakers. My advice, place a filter somewhere in the amp to scoop the mids if using passive guitar pickups, the input signal won't be flat, there is a bump in the mid range that you'll want to even out. Or if you end up with more then enough gain (which I doubt) you can add a whole tone stack, to do this though you might have to add another stage.
Also the advice on the capacitors is that you don't have to buy uprated capacitors unless the voltage rating will be exceeded. DF96 explained it very clearly. IF you are below the current rating, like using 450v caps and the B+ is only 300, after removing the tubes (load) from one channel and adding a SS rectifier the voltage might jump up to 400V. This is still under the rating but to be safe and not have them blow up you will want to "reform" them. I use high value resistors and step them down to limit the current.
Last edited:
@famousmockingbird
??
No no no, this thread is about Hi-Fi Single-Ended Class A tube amp and it belongs here.
My references are from the guitar tube amp world because I had experience only with guitar tube amps.
NFB, Pentode ,Triode, speaker Impedance, Electronics, Ohm's Law and EVERYTHING else are the same non the less.
I understand your post is mostly sarcastic but I still appreciate real info regarding what is the "agreed standard" in Hi-Fi world.
Care to add something knowledgeable to this discussion? I'd be happy to learn.
Yes, I'm definitely going to hook my guitar Cabinet to it at some point just get a taste because I am very familiar with this cab and how it sounds with SS and P-P guitar tube amp.
And yes, I am going to use my Les Paul through a distortion pedal and an eq after it to recreate a guitar preamp scooped sound characteristics.
But the main intent is to use this hi-fi amp mostly for clean music (not metal or rock) with my Polk RTi A3 full-range speakers.
??
No no no, this thread is about Hi-Fi Single-Ended Class A tube amp and it belongs here.
My references are from the guitar tube amp world because I had experience only with guitar tube amps.
NFB, Pentode ,Triode, speaker Impedance, Electronics, Ohm's Law and EVERYTHING else are the same non the less.
I understand your post is mostly sarcastic but I still appreciate real info regarding what is the "agreed standard" in Hi-Fi world.
Care to add something knowledgeable to this discussion? I'd be happy to learn.
Yes, I'm definitely going to hook my guitar Cabinet to it at some point just get a taste because I am very familiar with this cab and how it sounds with SS and P-P guitar tube amp.
And yes, I am going to use my Les Paul through a distortion pedal and an eq after it to recreate a guitar preamp scooped sound characteristics.
But the main intent is to use this hi-fi amp mostly for clean music (not metal or rock) with my Polk RTi A3 full-range speakers.
Last edited:
My mistake. You were bringing up guitar amplifiers a lot and you did mention only using one channel of the stereo amp. I made an assumption if one channel is only being used it is for a guitar application because most audio playback these days is mixed in stereo.
My intent isn't sarcasm. I was trying to add something knowledgeable......I apparently failed. Typical of me. :/
My intent isn't sarcasm. I was trying to add something knowledgeable......I apparently failed. Typical of me. :/
Don't get me wrong, I AM going to crank the nuts out of this amp for guitar also.
But when I'm not, it will be your typical Tube Hi-Fi Frank Sinatra amp.
I think I'm not the first to use SE hi-fi amps for guitar... I think people did it several times my age ago.
I'm here to learn.
Here is a simple question:
Why would anyone want a high damping factor and low distortion from a tube amp if it will sound exactly the same as a SS?
Last edited:
Because the goal for each device is the same, which is to output a larger more powerful version of the input.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.