Hello,
giving a try as I came in contact with a diy version of this amp and would like to possibly discuss with an owner of the real deal or diy versions who could share experiences, measurements and why not worthwhile improvements.
As rare as hens teeth?
giving a try as I came in contact with a diy version of this amp and would like to possibly discuss with an owner of the real deal or diy versions who could share experiences, measurements and why not worthwhile improvements.
As rare as hens teeth?
Thanks a lot, not many I see.
It's my first class A amp and very pleased with it.
Out of curiosity I measured some resistor values which measured differenttly in there from the schematics, a couple were measuring correctly out of the circuit and were put back in but I found and substituted a 1,2K resistor for a 120ohm resistor as per supplied schematic (at the cathode of upper valve section of the input cascode).
There is also what I think is a big mismatch in the output pcbs as pin 7 of the output valve appears NOT connected... I'd like to possibly figure this out too.
Will try to mod the fixed tape output to have volume controlled output for a subwoofer section..
It's my first class A amp and very pleased with it.
Out of curiosity I measured some resistor values which measured differenttly in there from the schematics, a couple were measuring correctly out of the circuit and were put back in but I found and substituted a 1,2K resistor for a 120ohm resistor as per supplied schematic (at the cathode of upper valve section of the input cascode).
There is also what I think is a big mismatch in the output pcbs as pin 7 of the output valve appears NOT connected... I'd like to possibly figure this out too.
Will try to mod the fixed tape output to have volume controlled output for a subwoofer section..
And be aware, the bias of the output tube is (intentionally) strongly influenced by the first triode. ECC88s are all over the place imo, DC wise, so keep an eye on that if you are thinking of tube rolling.
Thank you Gents for your suggestions.
Wondering why pin 7 was not connected.. I made a connection now b/w pin 7 and 2 and spatial information in the output sound seems much improved, for good! I am at the 4ohm output tap, will want to go back to the 8ohm to check if this would have been best in the first place with my speakers (Pawel Ensemble Reference monitors, which are specced at 4ohm, due to a couple of 4 ohm dips in impedance at low frequencies).
As for resistors (apart from an added 1k resistor at grid of first valve which is not noted on the schematic), the 10M value of resistor in parallel with 0,022uF cap is "only" 3,3M.. not sure what this does but wondering whether I am loosing some/too much bottom end.. will add a pair of 10M resistors in the next order in any case.. btw measured voltage of upper first valve cathode is about 150VDC whilst this should be 120-50VDC (from 70VDC to 120VDC iirc) according to specs.. also wondering whether the 10M resistor is participating to first valve bias?
And btw I have a couple of 6DJ8 and ECC88 which I will want to try.. not sure about @hbc 's warning and how pernicious this would be.. should I probably use dummy speakers if doing so?
Wondering why pin 7 was not connected.. I made a connection now b/w pin 7 and 2 and spatial information in the output sound seems much improved, for good! I am at the 4ohm output tap, will want to go back to the 8ohm to check if this would have been best in the first place with my speakers (Pawel Ensemble Reference monitors, which are specced at 4ohm, due to a couple of 4 ohm dips in impedance at low frequencies).
As for resistors (apart from an added 1k resistor at grid of first valve which is not noted on the schematic), the 10M value of resistor in parallel with 0,022uF cap is "only" 3,3M.. not sure what this does but wondering whether I am loosing some/too much bottom end.. will add a pair of 10M resistors in the next order in any case.. btw measured voltage of upper first valve cathode is about 150VDC whilst this should be 120-50VDC (from 70VDC to 120VDC iirc) according to specs.. also wondering whether the 10M resistor is participating to first valve bias?
And btw I have a couple of 6DJ8 and ECC88 which I will want to try.. not sure about @hbc 's warning and how pernicious this would be.. should I probably use dummy speakers if doing so?
As @hbc wrote, P/ECC88s are over the place how they bias up especially in such a current starved cascode; plate current is only 0.6mA and the lower input tube is working with just 30 ... 40V on the plate; that's way down in the crooked left bottom corner of the plate curves.
One channel measured 150V, the other 190V, if I remember this correctly, while a simulation showed as low as 100V.
In my simulation the cathode current through the EL509 varies quite a bit with these variations in the input stage.
Here's the numbers from sim with 6KG6 model (equivalent to original sweep EL509; Swetlana or JJ EL509s are somewhat different as are used ones) :
input stage bias, resistor, EL509 cathode current, total dissipation (plate + screen):
120V 10Mohm 70mA 30W
150V 10Mohm 80mA 35W
120V 3.3Mohm 120mA 50W
150V 3.3Mohm 150mA 60W
It seems that the previous owner tried to increase the output tube dissipation to the max by going from 10Mohm to 3,3Mohm.
But it is easy to calculate actual tube current by measuring voltage across the 250Ohm cathode resistor.
Ic = Vc / 250;
total dissipation (plate+screen combined) is then
Pd = (Vp - Vc) * Ic
SY wrote a long time ago
"I remember seeing a cascode circuit from Tim deParavicini that looked like it would be awful: ECC88s running at starved-current levels and a very high plate resistor. I built and measured it just for curiosity (Tim couldn't be THAT wrong, could he?) and was astonished at how low the distortion was (it's probably posted somewhere here on the forum).
In retrospect, I can at least get a hint about why this was true: the high value resistor on the top tube's plate provided a reasonable load for the bottom tube. There's a tradeoff between Miller capacitance reduction and linearity that he saw and I (initially) didn't."
Remember that TdeP was no dummy
"I remember seeing a cascode circuit from Tim deParavicini that looked like it would be awful: ECC88s running at starved-current levels and a very high plate resistor. I built and measured it just for curiosity (Tim couldn't be THAT wrong, could he?) and was astonished at how low the distortion was (it's probably posted somewhere here on the forum).
In retrospect, I can at least get a hint about why this was true: the high value resistor on the top tube's plate provided a reasonable load for the bottom tube. There's a tradeoff between Miller capacitance reduction and linearity that he saw and I (initially) didn't."
Remember that TdeP was no dummy
Thank you Gents, really appreciating your wisdom and inputs.
Measuring and calculating dissipation of power tubes with different input valves has been an eye opener, well considering my eyes were basically wide shut 😀
The driver valve seems NOT to affect biasing of power valves (it could well be it's just meant to be this way? makes sense).
Power valves are Russian 6P45S (assuming Svetlana) from '73.
I'll spare bunch of details, however came to following situation,
with 6DJ8 pair at input (this is the pair I own which has the best pairing between channels and has lower dissipation at power valve too) and with a 7DJ8 at driver, following values were measured and dissipation calculated,
input stage bias, resistor, EL509 cathode current, total dissipation (plate + screen), heater tension:
128/147V 3.3/3.3Mohm 85/88mA 41/42W 6,2VAC
A bit high on dissipation according to the specs, so what I did as a temporary solution is to wire the 230VAC power from the 220VAC tap of power transformer to the 240VAC tap. In this new scene,
input stage bias, resistor, EL509 cathode current, total dissipation (plate + screen):
130/150V 3.3/3.3Mohm 81/84mA 36/37W 5,6VAC
From @Sorento 's sim values the 10M should bring dissipation down (fingers crossed) and I should then be able to bring heater tension up to 6,2VAC for good!
(on a side thinking it might well have been that the previous owner used EL519 valves hence the choice of the 3,3Mohm resistor would make sense to burn more energy..)
Btw would the value of the 0,022uF parallel cap influence biasing too? or what is its value influencing?
Measuring and calculating dissipation of power tubes with different input valves has been an eye opener, well considering my eyes were basically wide shut 😀
The driver valve seems NOT to affect biasing of power valves (it could well be it's just meant to be this way? makes sense).
Power valves are Russian 6P45S (assuming Svetlana) from '73.
I'll spare bunch of details, however came to following situation,
with 6DJ8 pair at input (this is the pair I own which has the best pairing between channels and has lower dissipation at power valve too) and with a 7DJ8 at driver, following values were measured and dissipation calculated,
input stage bias, resistor, EL509 cathode current, total dissipation (plate + screen), heater tension:
128/147V 3.3/3.3Mohm 85/88mA 41/42W 6,2VAC
A bit high on dissipation according to the specs, so what I did as a temporary solution is to wire the 230VAC power from the 220VAC tap of power transformer to the 240VAC tap. In this new scene,
input stage bias, resistor, EL509 cathode current, total dissipation (plate + screen):
130/150V 3.3/3.3Mohm 81/84mA 36/37W 5,6VAC
From @Sorento 's sim values the 10M should bring dissipation down (fingers crossed) and I should then be able to bring heater tension up to 6,2VAC for good!
(on a side thinking it might well have been that the previous owner used EL519 valves hence the choice of the 3,3Mohm resistor would make sense to burn more energy..)
Btw would the value of the 0,022uF parallel cap influence biasing too? or what is its value influencing?
No, this cap has no influence on DC or bias.
The 10M together with the 3.3M resistor to ground form a voltage devider for DC bias.
They have to be high values in the Megohms to prevent excessive loading to the high impedance cascode output.
But such high values would also form a low pass RC filter with the input capacitance of the next stage.
In other words, severely limit the high frequency response.
To avoid this, the 10M resistor is bypassed with that parallel cap, allowing the AC signal to circumvent the 10M.
So, the whole thing is a voltage devider for DC but not for AC.
If the cap is too small however, it could compromize the low frequency response.
For the 10M/3.3M devider it is about adequate.
For a 3.3M/3.3M devider you could consider increasing its value to 0.033uF.
Hi, if by "driver" you mean the cathode follower driving G2, that should indeed have minimal influence on the output tube biasing. 🙂
Indeed @hbc that was my intention, guess I have not been 100% correct in my terminology? 😀
Been told about the previous owner that all the components have been present from the original kit, the build took place only few years ago.
I will want to install a 10Mohm resistor anyway and take it up from there to see whether biasing will be "more correct", as heater voltage to my untrained eye seems like a compromise.. as far as I can tell it is sounding well as it is in any case 🙂
As for 0,022uF in parallel with 10M resistor, would it be awful if I used a 0,047uF? (ehm.. happen to have a boutique cap laying around..)
Been told about the previous owner that all the components have been present from the original kit, the build took place only few years ago.
I will want to install a 10Mohm resistor anyway and take it up from there to see whether biasing will be "more correct", as heater voltage to my untrained eye seems like a compromise.. as far as I can tell it is sounding well as it is in any case 🙂
As for 0,022uF in parallel with 10M resistor, would it be awful if I used a 0,047uF? (ehm.. happen to have a boutique cap laying around..)
As I've advised geoturbo in a PM the best improvement I found was by exchanging the cathode follower for a mosfet source follower (SF). I built the SF into a 9 pin socket so I could easily swap from valve to SS. I made no bias adjustments. I and everyone who heard the comparison thought the SF was "better".
Just clarifying 🙂
I am not "recommending" this as a mod, but I ended up using a mosfet in this position on my 859 inspired build, (even though it is not on the circuit), as the cathode follower couldn't source enough current..
https://www.diyaudio.com/community/threads/ear-859-869-inspired-amp-build.384706/
Thank you Gents, keep 'em coming; I have not been able to make further changes nor order 10M resistors but I don't mind if you tell more about your improvements, have circuit diagrams been shared already maybe?
Thinking about improvements, do you reckon a regulator for B+ (Maida) at input valve be beneficial?
