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In Reply to: RE: Question on biasing MA-1 with DMM posted by Lew on December 09, 2023 at 20:03:35
Thanks Lew for offering your thoughts.
I understand the bias is measuring DC current. What confuses me is that the built-in meter is an amp meter used to measure both DC offset and bias and, to my knowledge, the meter is wired across the two speaker posts via the test/operate switch. One depresses the test/operate switch to engage the meter.
If one can measure the DC offset by engaging the amp meter or by measuring the voltage drop between the two speaker posts, why would it be different for the bias? I wonder how the meter is wired?
Follow Ups:
All MK3s have automatic bias so this is an older amp.
A DC Offset is generated when you depress the Bias Test switch. That switch disconnects one of the two power supplies in the Circlotron output section. This causes half of the amp to push DC though the load (in this case, the meter). Typically, the meter is such a vastly lower resistance than the DCR of the loudspeaker that you can leave the speaker in place while this is being done, and the DC isn't harmful to any speaker we've encountered. Older amps put the meter in series with the speaker and we found that there was a tendency to set the bias a little high.
So with a DVM set to DC Amps, you put it across the speaker terminals and then depress the Bias Test switch. If a 12-tube MA-1 you set for 650mA initially; after one hour it should nut exceed 700mA.
With a 14-tube version you set the bias to 700mA and do not exceed 820mA after one hour.
You can set the bias lower with less heat if you want. You may notice a loss of authority if you set it below about 80% of the values I've indicated.
Regarding the DC Offset adjustment: Do that first. The meter is connected the same way and all you're doing is setting the DC Offset control for as close to a zero reading as you can get. If there is a gross mismatch between the tube banks in the amp you won't have enough range and may have to juggle a couple of power tube back and forth until you're within the range of the control. The tube banks are either right hand or left hand on all MA-1s.
After you set the bias, allow the amp to sit for about a minute at idle and then check the DC Offset again. The two settings interact, so you might have to go back and forth a little to get things right.
When the amp is warmed up for an hour, usually you can just check the DC Offset and then set the Bias since the movements on the controls will be slight.
If you find the DC Offset drifts a bit, its an indication the driver tube (not the voltage amplifier tubes) is weak. On the 12-tube MA-1s this might be a 6CG7; if updated and on all the later amps its a 6SN7. Contact me if you don't know which tube this might be in your amp.
Many thanks Ralph.
My MA-1 is the 14-tube version with manual bias and 5 6SN7 in the input section. I believe it's the Mk 2.3 Silver version except I omitted the Class A/AB switch.
To the credit of the amp, the DC offset and bias are very stable once set and do not seem to drift much over time either. I've always used a DMM for adjustment with the speakers disconnected. I had not adjusted the amps for over a year and when I decided to check and adjust them I had forgotten the need to set the DMM to DC amp scale for the bias measurement. Thus my question.
Dunno. My amps have no built in meter, because they are wired so I can bias the output tubes individually. (There are only four per chassis, 7241s.) I still have the meters that came with, but they are in my parts bin. But Ralph will set you straight.
Edits: 12/10/23
Thanks again Lew.
I read with much interest about your 6C33C amp. I still have 4 or 8 6C33C tubes I bought probably 20 years ago hoping some day to build a amp that make use of them but it never came to fruition. Instead, I built many headphone amps that use EL34 ...
Ralph discontinued that particular model in the late 90s shortly after I bought my pair, in kit form from the factory. The MA240 used six 6C33Cs per monoblock, and it was essentially a 6C33C-based version of the MA2, used the same power transformers and chassis's. I have since converted them to use 7241s, but only four per monoblock. With Ralph's tutelage, I built the amps so that each output tube can be individually biased. Thus I needed to incorporate a separate dual triode driver tube for each pair of output tubes. I have updated and upgraded the amps several times in the past 25 years, but I cannot imagine being without them to drive full range ESLs.
Hi Lew:
I've never heard of 7241 tubes until you talked about them in your posts. I read the data sheet and gulp! But I gulped harder when I tried looking up how much these tubes cost...-
They have much more current capacity compared to a 6C33C, but each tube has 3 cathodes. So you are faced with the issue of either selecting tubes where the plate current to each of the 3 cathodes is well balanced (impossible due to cost and rarity) or limiting the bias current across the whole tube so as not to over-stress one or two of the three plate to cathode pathways. You also have to use current-sharing resistors on the cathodes, to protect against such an imbalance. Thus you cannot really take advantage of the enormous current capacity in terms of setting bias current much higher than for a 6C33C. (This limits Class A operation.) However, you do still get the very low plate resistance per tube, which is favorable for low output impedance. As Ralph knows, I had problems with blowing tubes until I learned to settle for a tolerable bias current per tube, which in my case is 285mA per tube. Another person uses the 7241 in his Novacrons, where he cannot separately bias each tube. He settles for 225mA per tube. OTOH, you can bias the 6C33C at 300mA per tube, at least in my amplifiers.
Hi Lew:
Changing of subject if you don't mind. I can open a new thread too if that's more appropriate.
I replaced the 5692 in my MP-1's voltage amp of the line stage with a new pair of KenRad VT-231. These KenRad's are microphonic as hell, I can hear the microphone noise as I turn the volume knob (Shallco switch). I am leaving them in place for the time being to assess their sound but wonder if I should get some tube dampers to try.
I remember you have posted similar issues in the past. Wonder if you have some thoughts and/or recommendations?
Best!
One correction, with the help of my wife, I confirm only the one KenRad VT-231 on the right channel is microphonic.
Any 6SN7, but it seems especially some of the best vintage ones, like the KenRad, is apt to be somewhat microphonic. I use ring dampers on the linestage voltage amplifier tubes in my MP1, for what it's worth. The 6SN7s in the circlotron output stage are less likely to be the problem, so far in my experience. Also, place the MP1 such that it is not likely to be "rattled" by the speaker output SPLs.
I just tried two more KenRad 6SN7GT tubes; one is also microphonic but fortunately one is not. I left the good one in the MP-1 for now. So, between the 4 KenRad NOS 6SN7 tubes, half of them are microphonic. Very disappointing.
Maybe I had been lucky, I've used many NOS 6SN7 tubes. Everything from 5692, VT231, GT, GTB, GTA and these two are the first microphonic ones. They are so bad, the noise is clearly audible 6 feet away from the speaker when all I did was turning the volume knob or lightly tap the pre-amp chassis, not even tapping the tubes.
It makes me wonder if it's the tube socket?
I have a few but have not used them for many years. I don't recall their being particularly microphonic. They are more robustly constructed, internally, compared to other 6SN7s, so I might have thought they would be less likely to ring. Where are you placing your preamplifier in relation to the speakers, and is the preamplifier well isolated from other sources of air borne and directly transmitted vibrations? All of that can make some tubes that are "mildly" microphonic, useable. I agree with Ralph that tube dampers don't help much, but the O-rings that I use seem to help at least somewhat. Currently running RCAs with the gray glass; I think VT231s.
Thanks Ralph and Lew. I can confirm that it's the tube that's microphonic and has nothing to do with the tube socket. It was just my wishful thinking that maybe I did not insert them firmly and tightly into the socket that contributed to the ringing noise.
The MP-1 is well isolated and has the full damping upgrade from the factory. These two KenRad tubes (one 6SN7GT, one VT231) are just badly microphonic - turning the Shallco volume pot was enough to make them ring. They do have very nice sound though so I am thinking about putting them in the V2 position of the MA-1.
Agree with the 5692 tubes being robustly constructed and not likely to ring. I actually remember reading somewhere they were made for military purposes and intended to be used in equipments situated at artillery base.
I've had that happen, where the clicking of the attenuator is enough to set off the tube. In the MP1,the pot is located close to the R channel linestage input gain tube, which makes for a problem in the case where the tube is microphonic to begin with. But in some cases, the tube stops ringing once you're set the gain to suit your taste.
Microphonic tubes do. Damping rings do very little to address this issue too. So the best place for such a tube is in an amplifier where there is less gain downstream.
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