Good work. These are over a half century old, so it's not surprising there are some problems.
I'm reading as recommended to remove C1. DC blocking cap.
I have a Croft micro 25 as preamp. Does this preamp have DC blocking so I'm ok to bridge the C1 cap on the dynaco. Can I do this mod without installing a choke?
I have a Croft micro 25 as preamp. Does this preamp have DC blocking so I'm ok to bridge the C1 cap on the dynaco. Can I do this mod without installing a choke?
Yes, all tube preamps will have an output coupling capacitor.
So you can eliminate the Dyna input capacitor without problems,
as long as the preamp (or turntable) has no subsonic low frequency instability.
The Dyna uses a 50 ohm resistor instead of a choke. Either way is ok.
So you can eliminate the Dyna input capacitor without problems,
as long as the preamp (or turntable) has no subsonic low frequency instability.
The Dyna uses a 50 ohm resistor instead of a choke. Either way is ok.
Something that is bugging me a bit. The tubes on one side of the amp are running so much hotter than the other. Although channel loudness is the same and even between left and right.
Is it something to be worrying about? Could it be a resistor on one side?
Is it something to be worrying about? Could it be a resistor on one side?
Check (when cold ) the single cathode bias resistor value in the cathodes. Should be 95 ohms.
Properly, these four tubes should be a matched quad, since there is only one common bias resistor.
Probably your four tubes are not closely enough matched, and some are running too hot.
Highly recommended is to separate the channels, and so have one cathode bias resistor per channel, 180 ohms 5W.
Use the hot pair of output tubes in one channel, and the other two output tubes in the other channel.
You must also separate the bias resistor's parallel capacitor ( each 100uF 50V ), one per channel.
So each channel has a cathode bias circuit of 180 ohm // 100uF . This will work much better.
Properly, these four tubes should be a matched quad, since there is only one common bias resistor.
Probably your four tubes are not closely enough matched, and some are running too hot.
Highly recommended is to separate the channels, and so have one cathode bias resistor per channel, 180 ohms 5W.
Use the hot pair of output tubes in one channel, and the other two output tubes in the other channel.
You must also separate the bias resistor's parallel capacitor ( each 100uF 50V ), one per channel.
So each channel has a cathode bias circuit of 180 ohm // 100uF . This will work much better.
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Matched EL84 is needed. That will fix your heat imbalance
Keeping the common cathode resistor will benefit loadsharing left<>right, when
one side needs more power the other side will actually help.
Keeping the common cathode resistor will benefit loadsharing left<>right, when
one side needs more power the other side will actually help.
so by having a matched quad the loads will be balanced. How it is now does not mean that something is out of range or broken. Apart from varing tube values.
So no need to worry i guess. I would like to keep the amp in original circuit.
what tube makes would you receommend for detailed warm sound?
So no need to worry i guess. I would like to keep the amp in original circuit.
what tube makes would you receommend for detailed warm sound?
The Original Circuit (kit or built) came with a very well matched set of output tubes.
Anything other than such well matched output tubes . . . you get what you are getting.
And, by the way . . . un-balanced output tubes is Not the original design ('original design' goes out the window, no longer accurately preserved)
Or, change to 4 individual self bias resistors, and 4 individual bypass capacitors. No longer original, but does not require quite as well of tube match (but match them anyway).
And now, you can easily see how badly your output tubes are un-balanced (The different voltages across all 4 self bias resistors). I = V/Rbias
95 Ohms, is that original? Then 95 x 4 = 380 Ohms for each of the individual self bias resistors.
You lose a little real estate, but you gain Finesse.
And, push pull output transformers will love you when you match the push and the pull currents!
Early Saturation will be gone!
Just my opinions, your mileage may vary.
Original to sell it?
Original to listen to (be sure to use very well matched ourput tubes, or it will not sound the same as the original).
Modified for ease of setup, checking tubes in circuit, etc.
Your choice.
I do not have any EL84 tubes, but if I needed some, I would get a very well matched Quad of JJ EL84 from Eurotubes.com
They ship overseas. (I drive to them for my other JJ tube type needs).
Have Fun!
Anything other than such well matched output tubes . . . you get what you are getting.
And, by the way . . . un-balanced output tubes is Not the original design ('original design' goes out the window, no longer accurately preserved)
Or, change to 4 individual self bias resistors, and 4 individual bypass capacitors. No longer original, but does not require quite as well of tube match (but match them anyway).
And now, you can easily see how badly your output tubes are un-balanced (The different voltages across all 4 self bias resistors). I = V/Rbias
95 Ohms, is that original? Then 95 x 4 = 380 Ohms for each of the individual self bias resistors.
You lose a little real estate, but you gain Finesse.
And, push pull output transformers will love you when you match the push and the pull currents!
Early Saturation will be gone!
Just my opinions, your mileage may vary.
Original to sell it?
Original to listen to (be sure to use very well matched ourput tubes, or it will not sound the same as the original).
Modified for ease of setup, checking tubes in circuit, etc.
Your choice.
I do not have any EL84 tubes, but if I needed some, I would get a very well matched Quad of JJ EL84 from Eurotubes.com
They ship overseas. (I drive to them for my other JJ tube type needs).
Have Fun!
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Matched pairs are still easy to buy, so if you plan to keep the amplifier, it's best to use a separate RC per channel.
The original circuit was designed "only" for matched quad tubes, which were common at the time, but not now.
The original circuit was designed "only" for matched quad tubes, which were common at the time, but not now.
Even if I swap position of the tubes I get warm tubes in the same sockets. Would this point to anything?
That is more questionable. There's no reason for that, other than leakage in the grid coupling capacitors.
If they are "black cat" types, this is typical. Probably should replace all 4 of those capacitors then.
Hard to say. Maybe add a 10 ohm resistor in series in each cathode and measure the voltage across each.
Or it could be your method of measuring temperature. Adding those resistors and measuring will tell.
Change the four coupling capacitors before installing the new tubes. The tubes could be damaged.
If you change capacitors, then give them a very large voltage margin.
600V are what I use on all my amplifiers.
Leakage?
What Leakage?
Of course, none of my amplifiers B+ ever comes close to 600V (gets close to 500V with cold tubes).
No coupling cap over-voltage under any - fixed tube bias, shorted grid/cathode, unloaded B+, etc.
Your Mileage May Vary
600V are what I use on all my amplifiers.
Leakage?
What Leakage?
Of course, none of my amplifiers B+ ever comes close to 600V (gets close to 500V with cold tubes).
No coupling cap over-voltage under any - fixed tube bias, shorted grid/cathode, unloaded B+, etc.
Your Mileage May Vary
I have come across another strange thing. The amplifier powers on ok. Plays fine then looses a fair bit of volume in one channel. Then when you switch it off the volume comes back in that channel during power down discharge for a second or two. It's bugging me.