Is latest revision ready for prime time? Jumpy bias on these amps drives me nuts, curious if these are are reliable enough now to assume control?
+5
Roy Mottram
Bob Latino
peterh
bluemeanies
LeGrace
9 posters
AB for M125?
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
bluemeanies- Posts : 274
Join date : 2015-02-09
Age : 73
Location : Folsom Pa.
I have the m125's without the AB control and I am using the Genalex Gold Lion KT88's..no jumpy bias here..Jim McShane..I highly recommend.
Had the Sovtek KT88's and the bias was jumping constantly.
So based on this I have to assume quality matched tubes are necessary.
Had the Sovtek KT88's and the bias was jumping constantly.
So based on this I have to assume quality matched tubes are necessary.
peterh- Posts : 1833
Join date : 2012-12-25
Location : gothenburg, sweden
It's tubes that is jumpy, not the amp.LeGrace wrote:Is latest revision ready for prime time? Jumpy bias on these amps drives me nuts, curious if these are are reliable enough now to assume control?
Get a matched quad from a reputable dealer and you will avoid "jumpy bias"
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
LeGrace wrote:Is latest revision ready for prime time? Jumpy bias on these amps drives me nuts, curious if these are are reliable enough now to assume control?
A steady bias reading is predicated by a steady line voltage. If your line voltage changes even a 1/4 of a volt, then the bias will change noticeably. In some parts of the USA and Canada the line voltage changes quite a bit during the day. As line voltage goes UP so will the bias. A rock steady bias voltage is not absolutely necessary for good sound. Output tubes have a "range" over which they will operate efficiently. The output tubes should be operated in the lower part of that range for good sound AND reasonably long tube life.
Bob
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
I've never had any trouble with steady bias on EL34 tubes, but many many times have had trouble keeping steady bias with KT88 tubes.
Make sure your bias grid resistors are 100K or less. If you ever run KT120 or KT150 those resistors need to be 51K. (R29-32 on any VTA board)
Make sure your bias grid resistors are 100K or less. If you ever run KT120 or KT150 those resistors need to be 51K. (R29-32 on any VTA board)
KSMITH- Posts : 6
Join date : 2019-04-14
tubes4hifi wrote:I've never had any trouble with steady bias on EL34 tubes, but many many times have had trouble keeping steady bias with KT88 tubes.
Make sure your bias grid resistors are 100K or less. If you ever run KT120 or KT150 those resistors need to be 51K. (R29-32 on any VTA board)
M-125 monos bias nicely with KT120 with no resistor change necessary. KT150s are a different story IME.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
KSMITH wrote:tubes4hifi wrote:I've never had any trouble with steady bias on EL34 tubes, but many many times have had trouble keeping steady bias with KT88 tubes.
Make sure your bias grid resistors are 100K or less. If you ever run KT120 or KT150 those resistors need to be 51K. (R29-32 on any VTA board)
M-125 monos bias nicely with KT120 with no resistor change necessary. KT150s are a different story IME.
Wish I could say the same. Since installing KT120's my M125 are hypersensitive. My target is 0.6 per tube. Quite common I'll turn the amps on and see a low reading on the order of 0.3. I'll barely move the pots and recheck and they jump to 0.8 to 0.9. Move them back a whisker and bias falls way too low again. Dialing them in is a nightmare. Doesn't help that my line voltage is up and down like a toilet seat.
The AB boards I'm hoping can be my salvation. Surprised though that no one has chimed in yet and said I've done this mod and they are great.
KSMITH- Posts : 6
Join date : 2019-04-14
My house has its own transformer. I do run a variac to reduce but my voltage is fairly steady throughout the day and night. The utility LTC seems to be set at 122 VAC with a deadband of +/- 3 VAC from my observations. Bottom line is my line voltage is very clean and solid. From what I read that is not always the case.
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
I thought that only existed in fairy talesKSMITH wrote:My house has its own transformer. I do run a variac to reduce but my voltage is fairly steady throughout the day and night. The utility LTC seems to be set at 122 VAC with a deadband of +/- 3 VAC from my observations. Bottom line is my line voltage is very clean and solid. From what I read that is not always the case.
Guest- Guest
I have no problem with bias - mine have been running strong for close to 8 years?
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
Wow, eight years, same tubes? Where's your bias set?costerdock wrote:I have no problem with bias - mine have been running strong for close to 8 years?
Guest- Guest
deepee99 wrote:Wow, eight years, same tubes? Where's your bias set?costerdock wrote:I have no problem with bias - mine have been running strong for close to 8 years?
Not same tubes - I got about 6 years out of my Sovtek KT88s - my favorite tubes.
I've since used KT88 Gold Lions - lost one - then replaced all with Sovtek 6550 which I'm very happy with.
I run two tubes in each amp with the bias at 500.
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
I think that bias setting is your secret. I keep mine at .475 and can't hear a bit of difference between that and the .62 maximum allowed (at least in VTA amps. Hopefully the payoff will be in longer tube and transformer life.costerdock wrote:deepee99 wrote:Wow, eight years, same tubes? Where's your bias set?costerdock wrote:I have no problem with bias - mine have been running strong for close to 8 years?
Not same tubes - I got about 6 years out of my Sovtek KT88s - my favorite tubes.
I've since used KT88 Gold Lions - lost one - then replaced all with Sovtek 6550 which I'm very happy with.
I run two tubes in each amp with the bias at 500.
Guest- Guest
deepee99 wrote:I think that bias setting is your secret. I keep mine at .475 and can't hear a bit of difference between that and the .62 maximum allowed (at least in VTA amps. Hopefully the payoff will be in longer tube and transformer life.costerdock wrote:deepee99 wrote:Wow, eight years, same tubes? Where's your bias set?costerdock wrote:I have no problem with bias - mine have been running strong for close to 8 years?
Not same tubes - I got about 6 years out of my Sovtek KT88s - my favorite tubes.
I've since used KT88 Gold Lions - lost one - then replaced all with Sovtek 6550 which I'm very happy with.
I run two tubes in each amp with the bias at 500.
Yeah - I used to do closer to .600 - then backed off from that - and was glad to hear Bob recommending 1.0 and .5 vs 1.2 and .6 (for M125 4 tube setup vs 2 tube setup - I prefer the 2 tube setup.)
I also had less rectifier issues.
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
Good thinking, if great minds think alike.
I ran my TS 6550s (old US-made) down to .450; anything below that and you could hear a little mud. .475 seems to be their 'company notch.'
I think I read somewhere that running output tubes at too-low of a bias is just as hard on them as over-biasing. Anyone else hear anything like that and explain why?
I ran my TS 6550s (old US-made) down to .450; anything below that and you could hear a little mud. .475 seems to be their 'company notch.'
I think I read somewhere that running output tubes at too-low of a bias is just as hard on them as over-biasing. Anyone else hear anything like that and explain why?
KSMITH- Posts : 6
Join date : 2019-04-14
deepee99 wrote:I thought that only existed in fairy talesKSMITH wrote:My house has its own transformer. I do run a variac to reduce but my voltage is fairly steady throughout the day and night. The utility LTC seems to be set at 122 VAC with a deadband of +/- 3 VAC from my observations. Bottom line is my line voltage is very clean and solid. From what I read that is not always the case.
400 amp service at the end of a dirt road....
corndog71- Posts : 840
Join date : 2013-03-19
Location : It can get windy here
deepee99 wrote:Good thinking, if great minds think alike.
I ran my TS 6550s (old US-made) down to .450; anything below that and you could hear a little mud. .475 seems to be their 'company notch.'
I think I read somewhere that running output tubes at too-low of a bias is just as hard on them as over-biasing. Anyone else hear anything like that and explain why?
I’ve been told that for longer life, power tubes should be run at half their max dissipation.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
Just received from Erhard-Audio:
https://www.amazon.ca/photos/share/6yJisLnNKx4fVlrFjxiHHhQJo6yPz3dPsgS74xJxgSu
The AB boards appear high quality.
Edit: Let me know if link works
https://www.amazon.ca/photos/share/6yJisLnNKx4fVlrFjxiHHhQJo6yPz3dPsgS74xJxgSu
The AB boards appear high quality.
Edit: Let me know if link works
New2Tubez- Posts : 184
Join date : 2018-03-20
Location : NY
LeGrace wrote:Just received from Erhard-Audio:
https://www.amazon.ca/photos/share/6yJisLnNKx4fVlrFjxiHHhQJo6yPz3dPsgS74xJxgSu
The AB boards appear high quality.
Edit: Let me know if link works
Link works (picture of 2 AB boards w/ caps, resistors and standoffs)
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
New2Tubez wrote:LeGrace wrote:Just received from Erhard-Audio:
https://www.amazon.ca/photos/share/6yJisLnNKx4fVlrFjxiHHhQJo6yPz3dPsgS74xJxgSu
The AB boards appear high quality.
Edit: Let me know if link works
Link works (picture of 2 AB boards w/ caps, resistors and standoffs)
Thanks for confirming. First time using Amazon photos. Tried to paste pic directly into post but didn't work out so this will have to do until I can figure it out.
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
No joy here on pics but they are nicely built.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
This weekend planning to start the build. Nervous but hopeful as well.
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
You'll do fine. Not speaking on his behalf, but Holger @ Earhart Audio is very handy with these things and do it right. Enjoy!LeGrace wrote:This weekend planning to start the build. Nervous but hopeful as well.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
Not an easy upgrade, really struggling on the first one. Installing all the wires onto the bottom of the AB board turned out to be a major mistake. Instructions say this is the neater option, more like the impossible option. Had to take a break got so frustrated. Will go at it again in the morning.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
Way less tricky installing the board first then attaching wires to top side. Still working on some issues but looking forward to benefits of this mod.
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