Well . . .
cooloox is smarter than some may think.
The schematic in his Post # 1 is very similar to his schematic in Post # 54.
Both output stages use "Ultra Linear Taps", even if the push and pull taps % are different from one side to the other on the donor output transformers.
The B+ may, or may not, be at the impedance center of the primary on the donor output transformer.
Both output stages are Self Inverting, with direct coupled cathodes, using a resistor (not a true high impedance current sink).
Any apparent difference in the output transformer (real difference, or not), would be to match the gain of the output tubes, to make up for the less than perfect current source (resistor) from the cathodes to ground (a different screen tap could make the effective gains equal between push and pull.
And, as noted, one g1 grid was at ground.
Of course it is . . . that is what enables the coupled cathodes to make the output stage self inverting.
(No phase splitter needed in front of the output stage)
If cooloox does not have proper measurement equipment, he will not be able to tell if the 'push' plate to B+ tap impedance is equal to the 'pull' plate to B+ tap impedance. And, he will not be able to determine if the B+ tap to the two screen taps are equal %.
If all that is available is an Ohmmeter, then . . .
The DCRs are not the best indicator of the primary turns,
but fairly close DCRs from the following will tell us something:
B+ tap to screen 1 tap, B+ tap to screen 2 tap
B+ tap to plate 1 tap, B+ tap to plate 2 tap
If they are within about 10 or 15% screen taps
and
within 10 or 15% plate taps . . .
Then the transformer primary is Traditional.
If the transformer primary is not traditional, then guess what . . . the unequal taps will improve the gain balance when using a less than perfect current sink (the resistor "current sink").
cooloox's request is to build a simple push pull, using his donor output transformers.
The discussions are great, but the intent is to help cooloox do that.
It is certain that to get enough gain to make the guitar signal large enough to drive the output stage anywhere close to its max power capability, a gain stage in front of the output stage is needed.
Post # 2 gain stage should be enough to get reasonable power out.
And, the solid state current sink in Post # 2 is optional (just use the resistor for the output cathodes).
We are not talking about a simple high end golden Hi Fi / Stereo amplifier here, are we?
Done.
Move on.
Build.
Listen.
Just my opinions, and I believe cooloox will be pleased.
P.S. I do not know where cooloox lives, but if he wants, I will donate, free, and send him a very well matched pair of JJ EL84 tubes, so he can build his project.
cooloox is smarter than some may think.
The schematic in his Post # 1 is very similar to his schematic in Post # 54.
Both output stages use "Ultra Linear Taps", even if the push and pull taps % are different from one side to the other on the donor output transformers.
The B+ may, or may not, be at the impedance center of the primary on the donor output transformer.
Both output stages are Self Inverting, with direct coupled cathodes, using a resistor (not a true high impedance current sink).
Any apparent difference in the output transformer (real difference, or not), would be to match the gain of the output tubes, to make up for the less than perfect current source (resistor) from the cathodes to ground (a different screen tap could make the effective gains equal between push and pull.
And, as noted, one g1 grid was at ground.
Of course it is . . . that is what enables the coupled cathodes to make the output stage self inverting.
(No phase splitter needed in front of the output stage)
If cooloox does not have proper measurement equipment, he will not be able to tell if the 'push' plate to B+ tap impedance is equal to the 'pull' plate to B+ tap impedance. And, he will not be able to determine if the B+ tap to the two screen taps are equal %.
If all that is available is an Ohmmeter, then . . .
The DCRs are not the best indicator of the primary turns,
but fairly close DCRs from the following will tell us something:
B+ tap to screen 1 tap, B+ tap to screen 2 tap
B+ tap to plate 1 tap, B+ tap to plate 2 tap
If they are within about 10 or 15% screen taps
and
within 10 or 15% plate taps . . .
Then the transformer primary is Traditional.
If the transformer primary is not traditional, then guess what . . . the unequal taps will improve the gain balance when using a less than perfect current sink (the resistor "current sink").
cooloox's request is to build a simple push pull, using his donor output transformers.
The discussions are great, but the intent is to help cooloox do that.
It is certain that to get enough gain to make the guitar signal large enough to drive the output stage anywhere close to its max power capability, a gain stage in front of the output stage is needed.
Post # 2 gain stage should be enough to get reasonable power out.
And, the solid state current sink in Post # 2 is optional (just use the resistor for the output cathodes).
We are not talking about a simple high end golden Hi Fi / Stereo amplifier here, are we?
Done.
Move on.
Build.
Listen.
Just my opinions, and I believe cooloox will be pleased.
P.S. I do not know where cooloox lives, but if he wants, I will donate, free, and send him a very well matched pair of JJ EL84 tubes, so he can build his project.
Last edited:
I live in estonia and I appreciate your kindness, 6A3sUMMER, let's discuss it when the need should come up. 🙂
As the website says:
Does it end here? Not yet!
The circuit above is wired for ultra linear configuration. (~10W)
If you tie g2 to B+, you have pentode configuration. (~15W)
If you tie g2 to plate, you have triode configuration. (~5W)
In other words, once you built this amp, you can really really really anal, and wire a switch to switch between different modes. How's this for 3 kind of sound in one package?
So if I wanted simplicity and power, I'd just forget about the UL and use the pentode conf.
About the type of the transformer, many soviet radios have an extensive service manual and I'm pretty sure it's available on the internet, the last SE radio I tinkered with had even descriptions how to rewind the output transformer in the manual. I possess a Rigonda 102 (mk1). So the manual could give the answers to the questions about the transformer(s).
I unfortunately don't have a lot of measurement equipment and the smartness part, there are people who would disagree. 😀
As the website says:
Does it end here? Not yet!
The circuit above is wired for ultra linear configuration. (~10W)
If you tie g2 to B+, you have pentode configuration. (~15W)
If you tie g2 to plate, you have triode configuration. (~5W)
In other words, once you built this amp, you can really really really anal, and wire a switch to switch between different modes. How's this for 3 kind of sound in one package?
So if I wanted simplicity and power, I'd just forget about the UL and use the pentode conf.
About the type of the transformer, many soviet radios have an extensive service manual and I'm pretty sure it's available on the internet, the last SE radio I tinkered with had even descriptions how to rewind the output transformer in the manual. I possess a Rigonda 102 (mk1). So the manual could give the answers to the questions about the transformer(s).
I unfortunately don't have a lot of measurement equipment and the smartness part, there are people who would disagree. 😀
The only “measurement equipment” needed to dope out that trafo is a DMM. The same one you use to test the B+, just set on AC. Drive the 8 or 16 ohm tap with the 6.3 volt output of a heater winding and measure all the primary taps. Voltage %’s will be accurate regardless of DCR differences and you can calculate Ra-a if that’s unknown. Just don’t touch anything when you power it up - unplug before moving the probes.
Any amp using a push pull pair will support 6.3 volts at 8 or certainly 16 ohms without overloading the trafo. That’s under five watts. If there is any doubt use a 3.15 volt center tap Or a small solid state amp to generate a volt or two.
Any amp using a push pull pair will support 6.3 volts at 8 or certainly 16 ohms without overloading the trafo. That’s under five watts. If there is any doubt use a 3.15 volt center tap Or a small solid state amp to generate a volt or two.
For a guitar amp, that's exactly the type of guitar sound I like! Ever heard of Ron Asheton of The Stooges?
Heck yeah. Ever hear of Destroy All Monsters? (Ann Arbor, 1980) 😀
I was thinking hi-fi, not guitar amps. For rock guitar (output stage overdrive) I would go pentode, not UL. UL is too polite.
Incidentally, Fender made a UL version of the Twin Reverb for a little bit around 1980 or so. I played through one once. Nice jazz guitar tone, smooooth.
I was thinking hi-fi, not guitar amps. For rock guitar (output stage overdrive) I would go pentode, not UL. UL is too polite.
Incidentally, Fender made a UL version of the Twin Reverb for a little bit around 1980 or so. I played through one once. Nice jazz guitar tone, smooooth.
Cooloox,
You said:
"The circuit above is wired for ultra linear configuration. (~10W)
If you tie g2 to B+, you have pentode configuration. (~15W)
If you tie g2 to plate, you have triode configuration. (~5W)"
That power output level may be true if you use a phase inverter to drive the output stage.
But with single ended driver stage sending signal to the self-inverter output stage will get less power than that.
However, if you build the self inverter output stage and simple driver; then you can get it up and running.
Then, you can listen;
Later you can put a phase splitter in front to drive the output stage, then you will get the additional power of non-self inverting push pull output.
You said:
"The circuit above is wired for ultra linear configuration. (~10W)
If you tie g2 to B+, you have pentode configuration. (~15W)
If you tie g2 to plate, you have triode configuration. (~5W)"
That power output level may be true if you use a phase inverter to drive the output stage.
But with single ended driver stage sending signal to the self-inverter output stage will get less power than that.
However, if you build the self inverter output stage and simple driver; then you can get it up and running.
Then, you can listen;
Later you can put a phase splitter in front to drive the output stage, then you will get the additional power of non-self inverting push pull output.
On the other hand, these days a lot of rock guitar players use little 'boutique' amps to get loud overdriven tone at reasonable volume levels. Just 5W into a typical >95dB/1W/1m sensitive guitar speaker can go loud enough to **** off the neighbors.
A self-inverting output stage is limited to class A operation, correct? I guess that means a pair of pentode mode EL84s driven full-tilt-boogie can still make 10W or so, no?
When you get into guitar amps optimized for overdriven tones, the clipping behavior becomes super-important. You want to cut the bass out so the amp doesn't 'fart out' on low notes, and you want to avoid blocking as much as possible. I don't know how well that can be done with a self-inverting output stage.
The classic guitar amps all used either LTP, cathodyne or paraphase phase splitters. Fender, Marshall, Vox, Gibson, Ampeg, Orange, Sound City, Kustom, Mesa Boogie, Traynor, etc. etc. I don't know of any that used a self-inverting output stage. Does anyone know of an example to share?
A self-inverting output stage is limited to class A operation, correct? I guess that means a pair of pentode mode EL84s driven full-tilt-boogie can still make 10W or so, no?
When you get into guitar amps optimized for overdriven tones, the clipping behavior becomes super-important. You want to cut the bass out so the amp doesn't 'fart out' on low notes, and you want to avoid blocking as much as possible. I don't know how well that can be done with a self-inverting output stage.
The classic guitar amps all used either LTP, cathodyne or paraphase phase splitters. Fender, Marshall, Vox, Gibson, Ampeg, Orange, Sound City, Kustom, Mesa Boogie, Traynor, etc. etc. I don't know of any that used a self-inverting output stage. Does anyone know of an example to share?
Last edited:
Guitar Amp:
Self inverting output stage:
To get a more distorted tone, use the 135 Ohm common cathode resistor as per schematic in Post # 1.
To get very smooth clipping, take out the 135 Ohm resistor, and use a constant current sink for the common cathodes.
Oh, I forgot to mention . . .
For a switch to change from Triode wired, Ultra Linear, and Pentode modes, Caution! Do not change the switch setting with the amplifier powered. Turn the amplifier off, let the B+ bleed down and than change the switch setting.
Self inverting output stage:
To get a more distorted tone, use the 135 Ohm common cathode resistor as per schematic in Post # 1.
To get very smooth clipping, take out the 135 Ohm resistor, and use a constant current sink for the common cathodes.
Oh, I forgot to mention . . .
For a switch to change from Triode wired, Ultra Linear, and Pentode modes, Caution! Do not change the switch setting with the amplifier powered. Turn the amplifier off, let the B+ bleed down and than change the switch setting.
From what I can recall, it was a Gibson Les Paul into a Marshall half-stack or something like that, and lots of attitude!
I can't think of a commercial one off hand, but the Firefly was pretty popular in the DIY space
https://ax84.com/archive/ax84.com/media/ax84_m276.gif
Based on Mullards 5-10 with handwound 0-4-8-16 ohms tap OPT, fully dual monoblock design, just completed. Runs dead silent on 105dB speakers:
(Sorry, not so great looking, it’s my first attempt at DIY chassis using only the Bosch hand drill. Have built many tube amps but all bare bones and this is the first fully dressed up)
Will be adding the 16 ohm terminal soon.
(Sorry, not so great looking, it’s my first attempt at DIY chassis using only the Bosch hand drill. Have built many tube amps but all bare bones and this is the first fully dressed up)
Will be adding the 16 ohm terminal soon.
With no other tool than a handrill not easy to accomplish what you did, congrats, nice work Yogi!
Pete
Pete
Last edited:
Thanks Pete. Building another. A single ended Pure Class A, super triode tube rectified 6BM8. Will share pictures soon.
Again, just the Bosch hand drill and jigsaw.
Again, just the Bosch hand drill and jigsaw.
You’re able to get those parallel slots that straight with a jig saw?? And not scrape the surface to death?
N
Those slots were already there on the top plate. It was intended to house a DAC but I decided to try it as the EL84 amp top plate.
But this would be changed too, to accommodate larger and better OPTs coming in soon. Better low end at higher wattage.
Oh no. Jigsaw is used on current 6BM8 build.
Those slots were already there on the top plate. It was intended to house a DAC but I decided to try it as the EL84 amp top plate.
But this would be changed too, to accommodate larger and better OPTs coming in soon. Better low end at higher wattage.
The self split output stage (resistor or CCS tail) does work with varying degrees of success in the HiFi amp world. I have never made one work great in a guitar amp. A self split output stage does need to work in or near class A. Any guitar amp, despite claims by the builder / manufacturer will leave class A as soon as the pedal board is set on KILL and the amp controls are set for the heart of the sun. The poor self split output winds up operating mostly as a SE stage or a flip flop because it becomes severely unbalanced. Operation becomes highly dependent on the type and condition of the output tubes.
The SPP is a good EL84 / 6BQ5 Hi Fi amp design, but a more typical 12AX7 LTP Marshall style phase splitter may be a better choice for a guitar amp, especially if it's going to be cranked to 11 a lot.
If lots of time is available, and some learning is desired, a potential budget guitar amp builder would do good to read through this entire thread:
https://www.diyaudio.com/community/threads/the-hundred-buck-amp-challenge.190738/
I made two guitar amps for that thread. The larger of the two was a Marshall inspired design with a series string UCC85 and UL84 tube set. The smaller was a quest for the absolute cheapest guitar amp that someone would actually use. Again, it used series string tubes that were, and still are $1 each. After some ugly flame wars as to what a "tube guitar amp" is it wound up with 4 tubes and a self split output stage. It worked, sounded OK, but I rarely used it.
Several years later I ripped it apart added a mosfet based "gyrator" load for the input tube, and a mosfet based split load phase inverter and a cathode bypass cap in the output stage to allow for class AB operation. The "gyrator" featured a gain control that could adjust the first stage for low gain clean tone, but it could be turned up to the edge of instability for some screaming lead tones. The "clean" (3% THD) power output went from almost 2 watts to over 4 watts, which is plenty loud in my basement. This little amp now sees lots of use. The schematic is here. Feel free to steal whatever ideas you want from it. The basic design should work with many different tube choices, as these are likely not found in Europe.
Keep it going Yogi, always good to see ppl making stuff emself instead of just buying
Latest build. (31st in last 3-1/2 years)
EL84 PP, tube rectified, 6SN7 at driver, no feedback. Interstage. Total 8 number of iron, 5 under the chassis. 9 watts per channel. 4, 8, 16 ohms. Dead silent. PSA-2N Electra Print Audio design. All iron handwound by Kolarkar Audio. Awesome tonality and details.