Go Back   Home > Forums > > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubelab Discussion and support of Tubelab products, prototypes and experiments

Reply
 
Thread Tools Search this Thread
Old 28th May 2020, 12:58 PM   #11
w5jag is offline w5jag  United States
diyAudio Member
 
Join Date: Feb 2007
Location: Arkansas
There was a thread a few years ago, that petered out after big holes had been smacked in an SSE board, and directly heated tubes installed, making a DHT SSE.

The thread literally went all over the place, but part of it discussed how to get better performance from the SSE front end at lower B+ voltages, part of which meant lowering the current in the CCS; 470 ohms was about as low as you can go as best I recall. The victim board also had LED's installed in place of the cathode bypass/bias resistor.

Maybe the fellow that built this board had been following that thread, but did not install the DHT's, or maybe he wanted to use lower voltage tubes like 6V6.

Tubelab SE: Removing MOSFETs?
__________________
Win W5JAG
  Reply With Quote
Old 28th May 2020, 09:05 PM   #12
RDL_AK is offline RDL_AK  United States
diyAudio Member
 
Join Date: Apr 2009
Location: Anchorage, Alaska
"The test board now has red LED's on the front end, in place of the resistor/bypass cap combination. I selected these on a trial and error basis. I took an arbitrary B+ voltage, measured the current at the cathode, then started swapping in LED's until I got a match. These nondescript red's turned out to be a good match. I changed the current set resistor in the CCS to 470 ohms"

"I'm using 25-30 year old red LED's for the 12AT7 bias on both boards. After reading up on this, I have no idea whether I went through the correct selection process. I didn't see any distortion difference between the LED and the RC combination; if anything, the LED might be slightly better. It sounds fine, so I guess I haven't botched anything. I'm wondering if a secondary artifact might be a visual indication of tube health and aging. "

Mr W5JAG I found these in posts 69 & 70 of your thread. Quite interesting reading.
How did you come up with the 470 ohm value?
So there is no real advantage of having the LED instead of the resistor and capacitor combo with a CCS?
  Reply With Quote
Old 28th May 2020, 10:43 PM   #13
w5jag is offline w5jag  United States
diyAudio Member
 
Join Date: Feb 2007
Location: Arkansas
Quote:
... How did you come up with the 470 ohm value?
Trial and error, using different CCS set resistors and then measuring the resulting distortion ( IMD ), at the reduced B+ voltage in use in the test amp.

Quote:
... So there is no real advantage of having the LED instead of the resistor and capacitor combo with a CCS?
They look neat, but, no, I could not measure or hear any difference. My test equipment is pretty mediocre, though. I think it is just a sideways move; others might have a different opinion or better data.

Both that DHT modified SSE and my daily driver SSE still have the LED's installed, for whatever that is worth.
__________________
Win W5JAG
  Reply With Quote
Old 28th May 2020, 11:23 PM   #14
RDL_AK is offline RDL_AK  United States
diyAudio Member
 
Join Date: Apr 2009
Location: Anchorage, Alaska
"You really have to measure the current, don't you? The CCS will do whatever it takes to put the preset current into the tube. The max plate voltage specified for the 12AT7 is 300. It's cathode biased, so you would subtract that. It's not really voltage that kills the tubes, it's power.

Tubelab could better answer your questions, but here is all I know about the 12AT7 driver stage, fwiw:

With a generic chinese 12AT7, and the stock compenents in the CCS, at 300 volts, the current into the CCS was 16 ma.

When I jumpered the dropping resistor, it only took 260 volts to achieve 16 ma through the CCS.

In looking at my notes, I don't see where I actually measured the voltage at the plate. I would expect that to vary tube to tube because it wants to deliver a specific current, not voltage.

With a 1 Khz sine wave into the CCS at 1 v p-p, the output sine wave does not clean up until there is at least 230 volts into the CCS. That got 40 v p-p out.

At 250 vdc into the CCS, 1.8 v p-p in, yielded a maximum 70 v p-p out before clipping.

At 300 vdc into the CCS, 3.8 v p-p in, yielded a maximum 140 v p-p out before clipping.

At 350 vdc into the CCS, 4.0 v p-p in, yielded a maximum 190 v p-p out before clipping.

I didn't see the IMD change much as I varied the voltage, but the load made a difference - it was lower into a Hi-Z load, by quite a lot, than it was into a 600 ohm load."

Mr W5JAG Thanks for your info. I seen this from post 3 of the linked thread. I attempting to understand the CCS function in the SSE with both stock RC and LED mod. That datasheet if am reading correct the max current is 15ma, typical 10ma and 7ma been min. Mr Tubelab stated the I will have only 5ma with the way this SSE setup. Is that too low? Is 16ma too High? I have done the jumper R14,R24 10k 3W resistors when using lower voltage for low power tubes before in a stock SSE. I could not hear the difference in that setup. Though I still do so because I think it makes parameters more correct for the 12AT7 and CCS. I am intrigued by your experiments and cake pan builds.
Attached Files
File Type: pdf IXCP 10M45S Data Sheet.pdf (520.6 KB, 2 views)

Last edited by RDL_AK; 28th May 2020 at 11:38 PM. Reason: Text
  Reply With Quote
Old 29th May 2020, 10:56 PM   #15
w5jag is offline w5jag  United States
diyAudio Member
 
Join Date: Feb 2007
Location: Arkansas
Quote:
... I will have only 5ma with the way this SSE setup. Is that too low? Is 16ma too High? I have done the jumper R14,R24 10k 3W resistors when using lower voltage for low power tubes before in a stock SSE. I could not hear the difference in that setup ...
As current in the CCS decreases, distortion increases. Lower current would probably extend the 12AT7 tube life if that is a concern. Lower current is necessary as the B+ voltage decreases, for the reasons stated in the linked thread. I can't hear the distortion difference at lower current, but some people might. My mediocre test equipment has proven to me that I do not hear distortion unless it is pretty gross.

I think both of my SSE are running at 470 or maybe even 560 ohms in the CCS because they are both running with modest plate voltage; they are at a different house than I am, so I can't just go take a look.

You can try it at 5 ma with the LED's and then try it with more current with the traditional cathode resistor / bypass capacitor and see if you can hear any difference. You're not going to hurt anything if you can solder / desolder properly. Maybe just change one channel so you can hear them side by side.
__________________
Win W5JAG
 

Reply


Some questions about mods made to SSEHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Questions about a circuit to made a IV amp filter tessier Digital Line Level 0 3rd May 2011 07:26 AM
Newbie questions - RH84 mods thevoice Tubes / Valves 16 10th December 2009 02:57 PM
Some basic questions about s-5 amp kit mods fenpark15 Tubes / Valves 15 22nd December 2007 10:56 PM
Questions for CD player mods DragonMaster Digital Source 2 13th November 2004 07:16 PM
Several questions about mods to the Leach Superamp craigt Solid State 5 6th April 2002 01:50 PM


New To Site? Need Help?

All times are GMT. The time now is 10:50 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki