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Mesa 400...6550/KT88?

#63
Thanks for your advice. I don't want to burn up anything, i will be xtra cautious, I checked the filament voltage. It seemed close to the 6l6
The 6550 & kt88 are running a little weak because of low plate voltage of the 6l6s
Trans seem cool??
 
#64
I just picked up a 12bh7 I want to see if it will work in the preamp section.
The first two preamp tunes are right off the input Jacks and before the eq and effects loop. the second two are after that and go straight to the power tubes. It looks like each set of two pp power tubes has its own cathode bias resistor so theroeticly I could really bias each individual set and push up the prformance of 2- 6550 even though the plate voltage is still low for the remaining 6l6s. I just have to mess with everything. It's a sickness.
 
#65
Bazz1111 said:
Thanks for your advice. I don't want to burn up anything, i will be xtra cautious, I checked the filament voltage. It seemed close to the 6l6
The 6550 & kt88 are running a little weak because of low plate voltage of the 6l6s
Trans seem cool??
Click to expand...
Heater VOLTAGE is not the issue, it's the current that can be an issue depending on the specific power transformer used. I do not recommend changing tube types without checking with the technical support guys in service first. Replacement power transformers are a very expensive part. Temperature of the transformer is not an accurate indicator as the core and copper are designed for the total load. It's entirely possible to overload one winding without a significant increase in temperature.

Plate voltage is only part of the reason, plate load is another important factor... a function of the reflected impedance through the output transformer.
 
#66
Bazz1111 said:
I just picked up a 12bh7 I want to see if it will work in the preamp section.
The first two preamp tunes are right off the input Jacks and before the eq and effects loop. the second two are after that and go straight to the power tubes. It looks like each set of two pp power tubes has its own cathode bias resistor so theroeticly I could really bias each individual set and push up the prformance of 2- 6550 even though the plate voltage is still low for the remaining 6l6s. I just have to mess with everything. It's a sickness.
Click to expand...
Whoa, the heater current reqt. for the 12BH7 is TWICE that of the 12AX/AU/AT type tubes. This is NOT a recommended substitute.

Note that the output stage is NOT cathode biased, I don't follow your logic or reasoning.

Modifying products is taken at your own risk, and the results could result in either a very costly repair, or possible an unrepairable amp. Especially important when considering the thoughts you have posted.
 
#68
Stock there are no cathode bias resistors in a 400+ or any mesa amp for that matter. Maybe someone installed some. The b+ in a 400+ is 500Volts at most so a 6550/kt88 wouldn't put out much if any extra power at that voltage. I've tested this several times on 400's, d-180's, m-190's. A 400+ is 12 6L6's and like aged horse said the output transformer is picked for that impedance not 6550's/kt88's. I'm sure if you just put the tubes in without anything else done its probably biased cold which is why it technically is working but the screen resistors aren't correct for those tubes, bias range probably isn't correct, there is no bias adjustment to begin with, 12 6550's cost how much??? I have a pretty extensive collection of tubes and I do not have 12 closely matched 6550's or even 2 sets of 6 that are matched very well.
You're asking for trouble. That amp isn't made anymore and for a while so not sure you'll be able to get a power transformer for it anymore.

If you want to play around with those tube types, get a 400 or d-180.
Not trying to be a jerk but I'm just trying to give advice so you don't blow up your amp.
 
#69
It is entirely possible to design an amp with a 500V B+ that can deliver pretty much full power with 6550's or KR-88's, but that would require an output transformer (and bias/support) specifically designed to the load line at that plate voltage. In fact, there are some advantages to doing so, but not pertinent to this particular discussion.

Nothing is as simple as swapping tubes, that's just one of several important (and interactive) variables.
 
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#76
Downunderwonder said:
General lack of reliability in modern production.

Moar better with milspec?
Click to expand...
Milspec what? There are different classifications depending on the application.

I have found that old or vintage tubes suffer from many of the same problems as current production tubes, old and vintage does not always mean better. There were PLENTY of junk tubes being made back in the day. The good 'ol days were nowhere near as good as often remembered.

I have a few hundred vintage tubes that I will sometimes use to test circuits with, the range of parameter deviation from the standards is quite wide, similar to current production tubes.

The mil specifications have different categories for things like mechanical, electrical, failure distribution, MTBF, corrosion, etc. Different applications will have different requirements.
 
#77
agedhorse said:
Milspec what? There are different classifications depending on the application.

I have found that old or vintage tubes suffer from many of the same problems as current production tubes, old and vintage does not always mean better. There were PLENTY of junk tubes being made back in the day. The good 'ol days were nowhere near as good as often remembered.

I have a few hundred vintage tubes that I will sometimes use to test circuits with, the range of parameter deviation from the standards is quite wide, similar to current production tubes.

The mil specifications have different categories for things like mechanical, electrical, failure distribution, MTBF, corrosion, etc. Different applications will have different requirements.
Click to expand...
if memory serves they were supposed to be happy at 500v and vibration proof.
 
#79
Agedhorse knows better than I do but a 400+ doesn't push the tubes super hard so any decent modern tube works well in them.
I've had good luck with the Ruby 6L6GCSTR which is the same tube as Mesa 440.
JJ's have a good following as well.
Just purchase from a good reseller and you'll be fine.
 
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