Hello All and thanks for the add.
I recently received 2 of these amps and I have re-capped both and set them up as mono-blocks per their instructions.
They sound pretty dang good to my ears.
My question; can I or should I change the EL34 tubes (I learned them as tubes, long time ago!) for KT66?
I have heard all kinds of stories, and viewpoints and I am just wondering it they would 'help' in any way?
The KT66s would be the Gold Lion which has some good review.
I would appreciate any info - just getting back into audio......
I recently received 2 of these amps and I have re-capped both and set them up as mono-blocks per their instructions.
They sound pretty dang good to my ears.
My question; can I or should I change the EL34 tubes (I learned them as tubes, long time ago!) for KT66?
I have heard all kinds of stories, and viewpoints and I am just wondering it they would 'help' in any way?
The KT66s would be the Gold Lion which has some good review.
I would appreciate any info - just getting back into audio......
Without someone having to re-invent the wheel, what are the operating voltages and currents of the EL34 tubes?
* 1:
For each channel, EL34 Plate voltage, plate current, parallel cathode voltage, and g1 bias voltage at A1 and A2?
What is the plate to plate primary impedance of the output transformer?
Are you willing to set the bias voltage until you get the proper current for each KT66 push pull pair?
(there might not be enough bias voltage, no matter where you set the potentiometer).
Are you willing to have about 6dB less loop gain versus the negative feedback setting?
Might be a good thing, depending on several factors, including your loudspeakers.
(the EL34 has about 2X the transconductance, and about 2X the g1/g2 mu; versus the KT66).
And, no matter whether you use EL34 or KT66 tubes, please be sure to use very very well matched pairs of output tubes. This amplifier does not allow any balancing of the Push and Pull quiescent plate currents.
Push Pull output transformers do not like un-balanced plate currents.
Let us know the answers to the measurements above (* 1🙂
Might be a very interesting modification.
* 1:
For each channel, EL34 Plate voltage, plate current, parallel cathode voltage, and g1 bias voltage at A1 and A2?
What is the plate to plate primary impedance of the output transformer?
Are you willing to set the bias voltage until you get the proper current for each KT66 push pull pair?
(there might not be enough bias voltage, no matter where you set the potentiometer).
Are you willing to have about 6dB less loop gain versus the negative feedback setting?
Might be a good thing, depending on several factors, including your loudspeakers.
(the EL34 has about 2X the transconductance, and about 2X the g1/g2 mu; versus the KT66).
And, no matter whether you use EL34 or KT66 tubes, please be sure to use very very well matched pairs of output tubes. This amplifier does not allow any balancing of the Push and Pull quiescent plate currents.
Push Pull output transformers do not like un-balanced plate currents.
Let us know the answers to the measurements above (* 1🙂
Might be a very interesting modification.
Wow, I had not realized I was that out of the tube-thought.
I do not (at this time) want to do much other mods than what I have done and I am not following even what you are asking - its been 40+ years since I did any tube work.
I was really hoping for a tube-switch and re-adjust for 2 green lights! <grin>.
What I really need is to go back and study your questions, and also perhaps find someone who has actually done this.
My understanding is these were one of the most manufactured (?) units so there has to be others who have modified their amps?
I am willing to re-learn and enjoy tube-sound once again with a little help from others.
I will re-read this again and start my studies.
Regards.
That is my primary concern as well as voltage from the supply.
Ecoli-557,
You never mentioned the manufacturer name nor the model number of your amplifier.
It looks very similar to my old Dyna Stereo 70, right down to the 2 negative feedback loops; the major difference is yours has the transistors and LEDs that indicate the proper sum of output tube current (but can not indicate the balance of those two tubes current; and the B+ rectifiers are solid state, not vacuum tube).
I know you wanted improvement for your amplifier. But you are happy with how it sounds now.
I considered you were also wanting to play with different modifications.
All kinds of things can be done to modify that amplifier.
Most will have lower power, some a little more distortion.
Individual self bias resistors and individual bypass caps for each output tube (balances plate currents; and reduces the plate to cathode voltage)
Another Choke added for Choke input filtering (much lower B+ voltge)
Triode wiring the output tubes (Many Pentode or Beam Power tube, some listed below).
KT66 output tubes; 6L6GC output tubes.
***
And, for the easiest modification of all . . . put in KT77 output tubes, and adjust the bias (a small adjustment; the amplifier should be able to give the right bias voltage for a KT77). Then plug and play.
EL34 Pentode, KT77 Beam Power tube. They have extremely similar specifications.
Hmm, EL34 on the left channel, KT77 on the right channel, and listen for the difference, than KT77 on left channel, and EL34 on right channel.
Again, all output tubes should be very closely matched for best performance (whether you use the fixed adjustable bias you have now; or whether you modify to individual self bias). By very closely matched, I mean each push to its pull tube. Does not have to be a Quad balanced foursome; just Left pair; and Right pair.
Whether you modify or do not modify, Have fun listening!
Oh, silly me, the thread title mentioned the amplifier name.
You never mentioned the manufacturer name nor the model number of your amplifier.
It looks very similar to my old Dyna Stereo 70, right down to the 2 negative feedback loops; the major difference is yours has the transistors and LEDs that indicate the proper sum of output tube current (but can not indicate the balance of those two tubes current; and the B+ rectifiers are solid state, not vacuum tube).
I know you wanted improvement for your amplifier. But you are happy with how it sounds now.
I considered you were also wanting to play with different modifications.
All kinds of things can be done to modify that amplifier.
Most will have lower power, some a little more distortion.
Individual self bias resistors and individual bypass caps for each output tube (balances plate currents; and reduces the plate to cathode voltage)
Another Choke added for Choke input filtering (much lower B+ voltge)
Triode wiring the output tubes (Many Pentode or Beam Power tube, some listed below).
KT66 output tubes; 6L6GC output tubes.
***
And, for the easiest modification of all . . . put in KT77 output tubes, and adjust the bias (a small adjustment; the amplifier should be able to give the right bias voltage for a KT77). Then plug and play.
EL34 Pentode, KT77 Beam Power tube. They have extremely similar specifications.
Hmm, EL34 on the left channel, KT77 on the right channel, and listen for the difference, than KT77 on left channel, and EL34 on right channel.
Again, all output tubes should be very closely matched for best performance (whether you use the fixed adjustable bias you have now; or whether you modify to individual self bias). By very closely matched, I mean each push to its pull tube. Does not have to be a Quad balanced foursome; just Left pair; and Right pair.
Whether you modify or do not modify, Have fun listening!
Oh, silly me, the thread title mentioned the amplifier name.
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Thanks for the return. I did indicate the name and model but only in the topic, probably not that noticeable. It is a Sound Valves VTA70i which is pretty close to the Dyna 70 I understand.
If I am to understand you, I can drop-in KT77s with no modifications? If that is the case, your suggestion of 'paired' tubes in only one channel and then roll them to the other channel is brilliant.
This is my 1st tube amp and I am just getting started......
What 6A3sUMMER said. It's a Dynaco clone. The output transformer impedance is too low for the KT66 to sound good, among other things. Stick with EL34, KT77 or 6CA7. LOTS of brands and different sound signatures to choose from.
A funny thing happens with Pentode and Beam Power output tube amplifiers which employ moderate to large amounts of global negative feedback:
If you do Tube rolling with tubes that have very similar specs (like the EL34, KT77, [and thanks to grovergardner for mentioning the 6CA7 which I forgot about]), then . . .
. . . Most often, all those tubes in that amplifier sound very similar. The global negative feedback tends to cover up most of the differences.
But if the tube plate currents are not very well matched on some of the tube-roll pairs you use, then there will be early saturation of the output transformers. That could make it sound different (global negative feedback does not fix output transformer saturation, it only makes it worse).
Changing from Pentode operation, to Ultra Linear operation, to Triode mode operation may give some different sounds, especially if you remove the global negative feedback.
Of course the power levels will be different from one mode to the other modes, but at low to low/mid power levels, there may be a different sound.
In general, Pentode mode almost always needs global or at least local negative feedback; Ultra Linear mode often needs global or an additional local negative feedback; and Triode wired mode can often work well with no global and no other local negative feedback.
Those results may vary widely, depending on your speakers.
For adjustable fixed bias like your amplifier is, always be sure to re-adjust the bias when you put another pair of output tubes in (even of the same manufacturer and same type number).
Your amplifier does have (should have) pin 1 and pin 8 connected together (that is for both EL34 and 6CA7 which connect the suppressor grid / beam formers to pin 1). The KT77 does not connect anything to pin 1, so the socket is wired for all 3 types of those tubes).
All Generalizations Have Exceptions
If you do Tube rolling with tubes that have very similar specs (like the EL34, KT77, [and thanks to grovergardner for mentioning the 6CA7 which I forgot about]), then . . .
. . . Most often, all those tubes in that amplifier sound very similar. The global negative feedback tends to cover up most of the differences.
But if the tube plate currents are not very well matched on some of the tube-roll pairs you use, then there will be early saturation of the output transformers. That could make it sound different (global negative feedback does not fix output transformer saturation, it only makes it worse).
Changing from Pentode operation, to Ultra Linear operation, to Triode mode operation may give some different sounds, especially if you remove the global negative feedback.
Of course the power levels will be different from one mode to the other modes, but at low to low/mid power levels, there may be a different sound.
In general, Pentode mode almost always needs global or at least local negative feedback; Ultra Linear mode often needs global or an additional local negative feedback; and Triode wired mode can often work well with no global and no other local negative feedback.
Those results may vary widely, depending on your speakers.
For adjustable fixed bias like your amplifier is, always be sure to re-adjust the bias when you put another pair of output tubes in (even of the same manufacturer and same type number).
Your amplifier does have (should have) pin 1 and pin 8 connected together (that is for both EL34 and 6CA7 which connect the suppressor grid / beam formers to pin 1). The KT77 does not connect anything to pin 1, so the socket is wired for all 3 types of those tubes).
All Generalizations Have Exceptions
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You have best use of your money if you buy music for he money. Replacing functional tubes
will be a no operation. Changing to incompatible tubes will of course have effect but
it will not improve the fidelity of the amp.
If you want new tubes , get a matched quad EL34 ( yes really matched ) from a vendor that
offers warranty.
Or get a quad of 6CA7 tubes. They are a drop in for the EL34 but as always you must rebias the amp. Find a quad of big bottle or fat boy 6CA7s and you’ll be happy. But, if what you have now is working just wait until they die. There are lots of brands and options to play with. Older NOS will likely sound better than new production but they are much more expensive.