Hi everyone. Output tubes are eight Sovtek 6550we in each mono block. There are accessible trim pots on the amplifiers for each tube and bias is through a 10 ohm resistor. What bias voltage should I be aiming for?
Thanks
Thanks
I "inherited" a pair of mono-block VTL parallel push pull EL34 amplifiers.
Someone put in some 6550 tubes, and the 10 Ohm cathode resistors burned out.
That is why I now have the amplifiers.
They are so heavy I have to pay for Hernia Insurance.
The VTL amplifier uses fixed adjustable bias, and the DC g1 total grid return resistance is about 470k Ohms.
They removed the EL34 tubes, and put in 6550 tubes, and the amplifier blew up.
Surprise, they had thermal run-away!
Plug and Pray.
Not Plug and Play.
Someone put in some 6550 tubes, and the 10 Ohm cathode resistors burned out.
That is why I now have the amplifiers.
They are so heavy I have to pay for Hernia Insurance.
The VTL amplifier uses fixed adjustable bias, and the DC g1 total grid return resistance is about 470k Ohms.
They removed the EL34 tubes, and put in 6550 tubes, and the amplifier blew up.
Surprise, they had thermal run-away!
Plug and Pray.
Not Plug and Play.
VTL MB-125
Use the following procedures to measure and adjust the bias setting:1.
Make sure that the amplifier is powered off. Remove the protective metal cage of the amplifier,
and make sure that there is sufficient room around the amplifier for you to access the bias
measurement points going across the front of the amplifier. Set the multimeter for resistance
measurement (ohms). (Consult the owner's manual of the multimeter to make sure that you
have used the correct settings.)
2.
Connect the negative (black, "-") probe from the multimeter to the negative("-") speaker binding
post of the amplifier.
3.
Consult the following figure for the locations of the bias measurement points on the amplifier's
top deck. Insert the positive (red, "+") probe of the multimeter into each bias measurement
point. The resistance reading on each point should be 10 ohms, within a 10% range. If the
resistance reading is outside the acceptable range, please consult the VTL factory customer
service department immediately. Repeat the measurement procedure for all 4 bias measurement
points.
4.
Remove the multimeter probes from the amplifier. Check to make sure that the amplifier is
connected to a loudspeaker or a load resistor. Either short the input or put the preamplifier in
the mute state (or turn the volume down) to make sure that no signal is coming from the
preamplifier. Power on the amplifier and let it warm up for about 10-15 minutes so that the
tubes become stabilized
5.
Change the meter setting to measure DC voltage. Insert the negative probe (black) from the
multimeter into the negative speaker binding post of the amplifier fitting it into the post where
the speaker cable is connected. Insert the positive probe into the bias measurement point of
each tube going from the #1 point to #4. The reading on the multimeter should stabilize after a
second or two and indicate a reading between 275 to 300 millivolts DC (0.275 to 0.300 Volts.)
This reading may fluctuate a little due to variations in the AC lines. It is not unusual for a 120V
rated AC line to vary between 115V and 122 V.
Caution: If the voltage reading is 50 millivolts or more below or above the acceptable
range then adjust the trimpot in the direction required and ensure that the reading is
changing. If the reading does not respond or continues to climb rapidly, or fluctuates with
large swings back and forth then you should turn off the amplifier immediately and
contact the VTL factory service department for assistance.
6.
In general a 10 - 15% variation around the 275-300 millivolts range is an acceptable bias
measurement.
When the measurement is outside this range, you will need to adjust the bias
setting by using the following procedures:
1.
Keep the positive probe from the millimeter in the same measurement point where you
found the out of range bias reading. Locate the bias adjustment point whose number
corresponds to the bias measurement point. See Figure 2 for the locations of the
measurement points.
2.
Insert a 1/8" flat tipped screwdriver (with a plastic shaft or a properly insulated handle) into
the bias adjustment trimpot which corresponds to the out of range measurement point.
Rotate the screwdriver slowly to see the changes on the multimeter reading. Try rotating the
screw in both directions to see the effect of the change. Adjust the setting of the bias to the
proper range, i.e. 275 to 300 millivolts DC.
3.
After you've completed the bias adjustment changes, it is a good idea to go back and check
the bias measurements for all the other tubes since changes made to one tube will slightly
affect the setting of the others.
4.
Place the amplifier's protective cage firmly back onto the amplifier after you are satisfied that
the measurements are stable.
https://www.manualslib.com/manual/1194626/Vtl-Mb-125.html#manual
I agree with rayma,
275mV to 300mV should be fine.
If all 4 "push" tubes read 275mV, and all 4 "pull" tubes are 300mV, then the current on the two halves of the output transformer will be un-balanced by 10mA (probably OK).
But if you match all the voltages more closely across the 10 Ohm resistors, the output transformer primary current balance will be better matched too.
275mV to 300mV should be fine.
If all 4 "push" tubes read 275mV, and all 4 "pull" tubes are 300mV, then the current on the two halves of the output transformer will be un-balanced by 10mA (probably OK).
But if you match all the voltages more closely across the 10 Ohm resistors, the output transformer primary current balance will be better matched too.
Thank you all for your comments. So are the bias values for the EL34 ( in the VTL-125) same as the 6550 WE tubes that is in the VTL MB250?
VTL has the same biasing procedure for all their tube amps, so that means set the reading
to the same 250 to 300 millivolts. However, only use the recommended tubes in their amplifiers.
https://www.vtl.com/support/tube-biasing/
to the same 250 to 300 millivolts. However, only use the recommended tubes in their amplifiers.
https://www.vtl.com/support/tube-biasing/
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Does anyone know if the VTL MB-250 originally came with EL-34s or 6550s?
There does not seem to be a manual or schematic available for it.
If the MB-250 came originally with EL-34 tubes, the 6550 could be incompatible with the amplifier circuit
because of too high grid resistance. The 6550 is rated for much less than the EL-34, in fixed bias mode.
There does not seem to be a manual or schematic available for it.
If the MB-250 came originally with EL-34 tubes, the 6550 could be incompatible with the amplifier circuit
because of too high grid resistance. The 6550 is rated for much less than the EL-34, in fixed bias mode.
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Try to find a copy of the venerable "VTL book" by Manley, there are a lot of infos about stuff, also for bias.
The standard value is 27 -30 mA , with 10 ohm resistor it will be 270-300mV
In every case is better to check visually inside the ampp on the pcb because in parallel with 10 ohm there is a cap; in some case, when the tube got a impulsive trouble, the cap become wrong. It is 220uF / 25-50 Vdc ( if aI remember well).
Walter
The standard value is 27 -30 mA , with 10 ohm resistor it will be 270-300mV
In every case is better to check visually inside the ampp on the pcb because in parallel with 10 ohm there is a cap; in some case, when the tube got a impulsive trouble, the cap become wrong. It is 220uF / 25-50 Vdc ( if aI remember well).
Walter
Although neither the 1988 original or the Old Colony second edition (edited by Paul Mazzarelli and probably superior) contains the MB250 model, some inferences can be made from the VTL books:
Like the Audio Research amplifiers of the era, B+ with contemporary American AC line voltages will be around 600VDC. Unlike the Audio Research's, G2 will also sit that high ("Ultra-Linear"). Not a huge issue when the Defense Department was dumping the MPD GE 6550s for chump change, but an issue now when only modern, inferior valves are available. 600VDC on an oxide cathode valve's G2 is serious duty.
Comparisons of grid circuit resistance ratings of valves made by the original manufacturers, back when they were an ordinary manufactured product, made in quantity by folks who did it every day, and had been doing it for years, who had access to proprietary materials and time tested techniques, to any current production valves, however well intentioned, is mistaken. There is zero correlation.
With all that as a preface, I've had decent luck with Jim McShane-selected modern "Genalex" KT-88's in big VTLs. 30mA is certainly reasonable; maybe even 35.
All good fortune,
Chris
Like the Audio Research amplifiers of the era, B+ with contemporary American AC line voltages will be around 600VDC. Unlike the Audio Research's, G2 will also sit that high ("Ultra-Linear"). Not a huge issue when the Defense Department was dumping the MPD GE 6550s for chump change, but an issue now when only modern, inferior valves are available. 600VDC on an oxide cathode valve's G2 is serious duty.
Comparisons of grid circuit resistance ratings of valves made by the original manufacturers, back when they were an ordinary manufactured product, made in quantity by folks who did it every day, and had been doing it for years, who had access to proprietary materials and time tested techniques, to any current production valves, however well intentioned, is mistaken. There is zero correlation.
With all that as a preface, I've had decent luck with Jim McShane-selected modern "Genalex" KT-88's in big VTLs. 30mA is certainly reasonable; maybe even 35.
All good fortune,
Chris