For someone expecting the forum to provide an exact “ordering response by Tuesday” you did not ask a precise question. What do you really want to know?
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Sorry but you are wrong.
The 6550 needs at most 50k grid resistor, the amp has 100k
KT88 will do with 100k
The down side with exceeding limits is that any problem overheating etc
might ( will ) cause grid current avalance, visible as redplating.
Many times this amp will work with 6550 for some time , but one is
on the wrong side of reliability wall. This is confirmed by users complaining
over redplating.
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iH8usrnames
Please feel free to correct me if I get this wrong...
First you ask if KT120's will work in you Bob Latino Engineered Dynakit amplifier and if memory is correct you were given an answer as to why they probably wouldn't be a good choice. Grid resistor mismatch was surely mentioned as well as , possible output transformer mismatch, and additional heater current draw. You seem to reject the sound advice given by long standing members here such as petertub.
You mention the Dynakit is the first tube amplifier you have owned and possibly that is why you figured you needed to run a variac instead of simply re-biasing the output tubes or possibly a modification to the power supply to lower your B+ and adjust your heater voltages so they would be inline with the standard specifications the amplifier was designed for.
A variac is generally meant for repair work instead of being an adjustable supply in which to power an amplifier. The fact that kids were around and able to play with the variac apparently unsupervised only adds icing to the cake.
You also seem to think that an transfusion to a different tube is going to correct the bass loss you are currently experiencing thru your selection of equipment.
I'm kind of curious here in that you state that your the manufacturer of this amplifier? No, you are the person that put a "Kit Amp" together since you didn't design a thing and you really didn't manufacture anything. You simply followed someone else's instructions possibly with help.
Manufacturing is the making of goods by hand or by machine that upon completion the business sells to a customer. Items used in manufacture may be raw materials or component parts of a larger product. The manufacturing usually happens on a large-scale production line of machinery and skilled labor.
My suggestion would be to contact Latino and mention first of all that your line voltage is a tad high and would he suggest a modification to the power supply to correct this.
Secondly, If interested in going with KT120's what modifications would he suggest or would he favor sticking to something more like the amplifier was originally designed for the KT88 and please do come back and let me know of his comments.
Please feel free to correct me if I get this wrong...
First you ask if KT120's will work in you Bob Latino Engineered Dynakit amplifier and if memory is correct you were given an answer as to why they probably wouldn't be a good choice. Grid resistor mismatch was surely mentioned as well as , possible output transformer mismatch, and additional heater current draw. You seem to reject the sound advice given by long standing members here such as petertub.
You mention the Dynakit is the first tube amplifier you have owned and possibly that is why you figured you needed to run a variac instead of simply re-biasing the output tubes or possibly a modification to the power supply to lower your B+ and adjust your heater voltages so they would be inline with the standard specifications the amplifier was designed for.
A variac is generally meant for repair work instead of being an adjustable supply in which to power an amplifier. The fact that kids were around and able to play with the variac apparently unsupervised only adds icing to the cake.
You also seem to think that an transfusion to a different tube is going to correct the bass loss you are currently experiencing thru your selection of equipment.
I'm kind of curious here in that you state that your the manufacturer of this amplifier? No, you are the person that put a "Kit Amp" together since you didn't design a thing and you really didn't manufacture anything. You simply followed someone else's instructions possibly with help.
Manufacturing is the making of goods by hand or by machine that upon completion the business sells to a customer. Items used in manufacture may be raw materials or component parts of a larger product. The manufacturing usually happens on a large-scale production line of machinery and skilled labor.
My suggestion would be to contact Latino and mention first of all that your line voltage is a tad high and would he suggest a modification to the power supply to correct this.
Secondly, If interested in going with KT120's what modifications would he suggest or would he favor sticking to something more like the amplifier was originally designed for the KT88 and please do come back and let me know of his comments.
iH8usrnames,
I re-read your first post and your later comments and my advice is to head over to the VTA forum and search for answers to some of your questions there. There are some VTA owners on this forum, like petertub or myself (M125 mono blocks), but you should be asking Latino directly for answers not already on that forum.
Latino actually recommends using variable transformers for these amps to trim high line voltages, but based on the typical voltages in many areas which exceed his recommended limit, I think he should redesign the power transformers to either provide taps for different line voltages, or simply to supply the correct voltages with today's line supplies.
My M125 mono blocks are running KT120 tubes and the transformers get very warm after an hour or so of operation, and it appears to me the margin is insufficient. I also use a bucking transformer that drops either 5 or 8 volts (switchable), which helps, but why not just design the PT for higher line voltages? I have considered having some wound and doing a complete rebuild into a larger chassis to space the big tubes further apart for more cooling. I made a number of safety, operation, and performance upgrades to my amps when I built them because I wasn't satisfied with the standard design and some of the parts.
My point is that since you admitted your lack of experience with tube amps, you are going to find out that there is a heck of a lot of important information you lack at this time. Why not go straight to Latino with your questions about tube selection?
I haven't searched the VTA forum exhaustively, but found only that he recommends different bias settings for the different tubes and amps (it's a sticky), but nothing about grid resistor change, which is a very valid concern/recommendation that you should discuss with Latino or post to the forum there. Not a lot of folks here have the specific knowledge of these amps or his design/operation approach, some of which is a bit unconventional, at least in regards to permanently deploying a Variac, which is something I will not do.
I re-read your first post and your later comments and my advice is to head over to the VTA forum and search for answers to some of your questions there. There are some VTA owners on this forum, like petertub or myself (M125 mono blocks), but you should be asking Latino directly for answers not already on that forum.
Latino actually recommends using variable transformers for these amps to trim high line voltages, but based on the typical voltages in many areas which exceed his recommended limit, I think he should redesign the power transformers to either provide taps for different line voltages, or simply to supply the correct voltages with today's line supplies.
My M125 mono blocks are running KT120 tubes and the transformers get very warm after an hour or so of operation, and it appears to me the margin is insufficient. I also use a bucking transformer that drops either 5 or 8 volts (switchable), which helps, but why not just design the PT for higher line voltages? I have considered having some wound and doing a complete rebuild into a larger chassis to space the big tubes further apart for more cooling. I made a number of safety, operation, and performance upgrades to my amps when I built them because I wasn't satisfied with the standard design and some of the parts.
My point is that since you admitted your lack of experience with tube amps, you are going to find out that there is a heck of a lot of important information you lack at this time. Why not go straight to Latino with your questions about tube selection?
I haven't searched the VTA forum exhaustively, but found only that he recommends different bias settings for the different tubes and amps (it's a sticky), but nothing about grid resistor change, which is a very valid concern/recommendation that you should discuss with Latino or post to the forum there. Not a lot of folks here have the specific knowledge of these amps or his design/operation approach, some of which is a bit unconventional, at least in regards to permanently deploying a Variac, which is something I will not do.
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Joined 2009
Paid Member
I’ve always been a bit skeptical of this Mr. Latino guy since observing that he sells his SP14 as the greatest thing since sliced bread but I can’t see where he gives due credit - the design is Broskie’s AIKIDO.
Changing an output tube's g1 resistor from 100k to 50k will double the load on the driver tube.
Find out what is the affect of doubling the load on the driver, in terms of the driver distortion, gain, and maximum output voltage before clipping.
Also, if RC coupling is used from the driver to the output tube, then the low frequency rolloff will change, and the phase angle will change too.
Investigate what this may do to low frequency stability for any amplifier that uses global negative feedback.
Every single design change usually requires 3 or more other changes in the amplifier just to make everything correct again (assuming that it ever was correct in the first place).
just sayin'
Find out what is the affect of doubling the load on the driver, in terms of the driver distortion, gain, and maximum output voltage before clipping.
Also, if RC coupling is used from the driver to the output tube, then the low frequency rolloff will change, and the phase angle will change too.
Investigate what this may do to low frequency stability for any amplifier that uses global negative feedback.
Every single design change usually requires 3 or more other changes in the amplifier just to make everything correct again (assuming that it ever was correct in the first place).
just sayin'
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Joined 2009
Paid Member
Actually, I asked the following:
I could swing a set of GL KT88's, or a set of 6550WE's and a GZ33 without much concern but, I'm not sure what the benefits of either decision would be?
1. I never mentioned Dynakit.
2. I am running a variac because I get more than 125v at my outlets very frequently - I am less than 100' from the transformer servicing my house and those around me. Bob Latino suggested running a variac to adjust the voltage to 120v.
3. As for kids being "icing on the cake"? I honestly need to hold back my opinion.
What I said was:
As of now the ST120 is delivering very little bass due to the 70hz crossover I set by Dirac to integrate my subs. I have, at times, disabled the subs and driven the Tritons full range but only for a few minutes at a time. The bass output is impressive, the speakers and amp, but I have not done so at length.
I made ABSOLUTELY no mention of bass loss.
Did you even read what I wrote?
Calling myself the "manufacturer" was a facetious statement. I followed that statement by noting it was a kit I assembled. I figured that would be enough for any reasonable person to understand that it was a kit provided by Tubes4Hifi.
"I am the manufacturer; I built it from a kit provided by Tubes4hifi."
He is the one who, in his documentation, notes the amp should not be used with voltages exceeding 125v. He further suggests using a variac to tune the voltage down to an acceptable range.
Well, I already know from his forum, people run 6550WE, KT88, and KT120 tubes without issue. Interestingly, I NEVER BROUGHT UP KT120 TUBES!
Look at post #14.
"First, I did not come here asking if it supports KT120 tubes."
As for letting you know ANYTHING, you clearly do not have the reading comprehension to read or reply to pretty much anything I have posted thus far.
First thing I did when I had the "Red Plate" was search his forum for additional information. I am satisfied this was either a fluke or the result of a toddler turning the big shiny knob.
I do not disagree, it would be wise to redesign the power transformer but even if he does I still have what I have and would still use the Variac.
I've done additional reading and found several people running KT88 and KT120 tubes, however, I never considered the KT120 tube. Was not even on my radar.
Actually, I am interested in building a new chassis to spread out the components and would also like to integrate bias meters.
The question I posted here is also posted on his forum, nearly verbatim. I thought this forum may be a bit more active thus offering more responses and variety.
Ultimately, this who post was about the option of running 6550WE or KT88 tubes, however, I offered more backstory than required which lead to confusion of the actual purpose of the post.
Thank you for your response.
I am trying to learn more, this is the only one I have to work with at the moment.
I've read "Vacuum Tube Amplifier Basics" cover to cover, once. I intend to do so again.
As a newbie, this amp seemed to be a very logical place to start. Seems to be a very well respected kit with quite a following. That is not to imply it is the single best amp the world has known, I am extremely pleased with it but I have nothing to compare it to.
One day I hope to read this and understand what you just wrote. 🙂
Honestly, I have tried finding somebody locally (outside Chicago) I could spend time with, learning. Obviously, pandemic hinders that as an option.
From what I gather, tube rolling is not as simple as changing a tube and adjusting bias - a load of other stuff must be accounted for.
Frankly, this thread is enough to turn me off to it altogether. Just get 6550WE, never bother with asking a question on a forum again.
Yes, I understood you were not looking at KT120 tubes. I confused you by mentioning my amps with their tube configuration, but I was trying to provide my amps as an example and make the point that there are variables and effects from the tube change you are considering, and had no idea you had already posted to the VTA forum.
I still think you would be best served by simply emailing Bob or calling him. Good luck.
I still think you would be best served by simply emailing Bob or calling him. Good luck.
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Joined 2009
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6550WE red plate
Means either the amp was faulty in it’s design or something in the amp failed or the tube was bad.
petertub provided you a gem, pointing out a potential design weakness of the amp - the grid resistor is too high for a 6550. This is a risk of thermal runaway which is why the tube manufacturer recommends a maximum value not to be exceeded. A particular set of 6550’s might do fine for years despite this, especially if the mains voltage tends to be lower than normal. Or one or more of the tubes may just be close enough to to the edge and red plate one day and whilst there are other reasons this may happen it makes sense to avoid a design weakness in the amplifier that increases the likelihood of this failure mode. The fact that the amp ships with 6550’s does not mean they are being operated within their working limits regardless of your confidence in the designer - check the tube manufactuers datasheet (example snippet attached) - it could be a marginal design and you were unlucky.
This design weakness can be corrected, but that means re-designing the amp and as pointed out by 6A3summer, it’s going to require some thought and work and this is something you may want to avoid until you are more experienced.
Therefore, you received some good advice that you are best off switching to KT88. There are many good brands to choose from and although I have no direct experience of KT88’s I’d recommend JJ KT88.
It does take some experience to get the best out of the forum - it takes two to tango so it ain’t just the other fellas.
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actually, just looking at his forum and found this:
VTA ST-70, VTA ST-120 and VTA M-125 amplifier information
It turns out he has updated the transformer but after I bought my kit.
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that’s good news, updating the transformer, if they’re same size etc. might be relatively easy to do
It’s possible other owners have seen red plating, hence the reduced bias etc. Doesn’t address the underlying cause thiugh.
It’s possible other owners have seen red plating, hence the reduced bias etc. Doesn’t address the underlying cause thiugh.
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