[crooner]

It's been one year since I ordered my DAC60 from Danny. I'm very happy with my purchase. Replacing the coupling caps with Sonicaps made a big difference in the sound.

Last year I experimented with a pair of Amperex Bugle Boy 6JD8s from the early 60s. They were fine but ultimately I found the sound rolled off on top with limited resolution.

I put the stock Electroharmonix 6922s back in. Recently I added some tube dampers on them. Don't know if they really need them, but they were cheap to try.

I was wondering if other fellow DAC60 owners got the Electroharmonix tubes as well. I believe I read somewhere that some folks were getting Sovtek tubes with the unit. They are similar but the EHs should be of a higher grade.

Anywho...

I've just took the plunge and bought a pair of Amperex white label PQ 7308 Gold Pin tubes from Upscale Audio. I bought the "driver/DAC grade" which should be fine with the DAC60.

I've also ordered pairs of JJ 6922s (steel pins) and Philips ECG JAN 6922s from the 80s.

I plan to do a shootout, even though the Amperex 7308 will probably win.

I've heard good things about the JJs and the Philips JAN. Perhaps I should use these tubes for a while and leave the expensive 7308s for special ocassions...

Is it really true that the DAC60 pushes the tubes hard? I've read this a few times. Hard to believe given their relatively easy task as buffers at unity gain. Perhaps I should take out the EH6922s and test them on my B&K 707.

I'll keep you guys posted on my findings as soon as I get the tubes!

[JoshK]

I hadn't heard they ran the tubes hard, its possible.  I'll have to check the schematic to see how much (quiescent) current they are running at.  I know they run at fairly low voltage.  The stage however isn't unity gain.  Its an SRPP or mu follower, can't remember which, which means two sections are used in each channel, one for gain one for unity buffer. That is why there are two tubes.

I haven't rolled tubes in it yet.

[crooner]

Thanks for your reply.
You are right about the stage not being at unity gain. Makes total sense.
Now, the 6DJ8 family is usually chosen for this application because it can operate on lower plate voltages, compared to the 12AX7. This results in savings on the power supply due to the lower B+ required.

I'd love to have an schematic of this. Let us know what you find about the stage's design.

Kevin Deal at Upscale recommends the lower "driver" grade of tubes for this application. I hope the ones I'm getting work ok in the DAC60. I might add some damping rings on them, just in case...

[audioengr]

Plate voltage is 180VDC.  Its high for this tube IMO.  Many designs run at 100VDC.  I like Siemens cca's the best, but they can die in 4-5 months depending on age etc...  The Amperex white-label 7308 PQ's are very good too.  Amperex PQ 6922's are supposedly good too, but have not heard them.

[crooner]

Interesting! Thanks for researching this.

I am now a little weary of using the Amperex 7308s as daily drivers. Even though they are rated at 10,000 hours. Perhaps I should try the Teslas or the Philips first.

[crooner]

According to the specs, the standard 6922 is rated at 130 volts max plate voltage. I wonder if there's a way to reduce the plate voltage on the DAC60 without impairing performance. Perhaps by changing the value of a dropping resistor in the power supply.

Apparently, the Sovtek/EH 6922 is rated for 220 V max plate voltage so this one should be ok in the circuit.

The 7308 is supposed to be a higher rated version of the regular 6922. I gotta check the specs on this one to make sure...

[tanchiro58]

I have listened to Amperex PQ 6922 and like their sound more than Siemens or Telefunken CCa. But they are microphonic. I just bought DAC 60 and will try the gold pins RCA mad in Germany.  :thumb:These tubes sound more mellow than Amperex PQ.

Good luck,
Tan

[crooner]

Hi Tan,
Thanks for replying.
Yeah they are microphonic, but usually in preamp or phono stage circuits. Let us know if you hear any microphony on the DAC60. One can use the tube damping rings if this is an issue, though...

[JoshK]

Plate voltage is 180VDC.  Its high for this tube IMO.  Many designs run at 100VDC.  I like Siemens cca's the best, but they can die in 4-5 months depending on age etc...  The Amperex white-label 7308 PQ's are very good too.  Amperex PQ 6922's are supposedly good too, but have not heard them.

Correct me if I am wrong but if the plate voltage is 180VDC and the stage is totem pole then the voltage is divided between the two tubes making 90VDC on each section's plate.  I am on vacation and don't have the schematic in front of me, but I think that is where I got the 90VDC idea in my head.

[crooner]

I certainly hope this is the case. Even though I tried using some clear ARC damping rings on the 6922 EHs once and they literally melted away. -Those tubes must be running a tad hot - I thought!

I can always stick my Fluke VOM in there and measure the actual plate voltage on each tube...

Correct me if I am wrong but if the plate voltage is 180VDC and the stage is totem pole then the voltage is divided between the two tubes making 90VDC on each section's plate.  I am on vacation and don't have the schematic in front of me, but I think that is where I got the 90VDC idea in my head.