Hello All.
As time passes, there are fewer and fewer DHTs left, and matching them becomes progressively more difficult. Thus my questions...
1. How matched do a pair of DHT power tubes need to be in a single ended triode amp?
2. What is the audible result of using a mismatched pair of DHTs in a single ended amp where one DHT power tube is used per channel?
3. If mismatched DHT output tubes result in reduced SQ, what is the best way to match them in circuit?
I Googalized this but found very little information of consequence. In my case it is 45 and 71A tubes that are most relevant, but I think this would apply to any DHT, including DHT preamp tubes. I have a number of old 45s and am in the process of building a 45 amp (The Bugle with a different power supply), but I only have one matched pair (TJ Full Music 45/n). Any advice or links that you can provide would be much appreciated.
As time passes, there are fewer and fewer DHTs left, and matching them becomes progressively more difficult. Thus my questions...
1. How matched do a pair of DHT power tubes need to be in a single ended triode amp?
2. What is the audible result of using a mismatched pair of DHTs in a single ended amp where one DHT power tube is used per channel?
3. If mismatched DHT output tubes result in reduced SQ, what is the best way to match them in circuit?
I Googalized this but found very little information of consequence. In my case it is 45 and 71A tubes that are most relevant, but I think this would apply to any DHT, including DHT preamp tubes. I have a number of old 45s and am in the process of building a 45 amp (The Bugle with a different power supply), but I only have one matched pair (TJ Full Music 45/n). Any advice or links that you can provide would be much appreciated.
In my experience first thing to assess with DHT's like 45 and 71a is if they are truly new or used. And if used, how much life is left. That's the major source of mismatch. Otherwise being low mu and moderate gm is rather easy to match them well enough to be fine (say, within 5% for current and gm, one to each other).
NOS 45's can still be found rather easily but price typically starts from about $80 each.
In case, as you have the (rather expensive) TJ you might also consider other current production 45's like Emission Labs and KR that are true replicas built with modern technology features. The TJ looks appealing but not a true 45, IMHO.
NOS 45's can still be found rather easily but price typically starts from about $80 each.
In case, as you have the (rather expensive) TJ you might also consider other current production 45's like Emission Labs and KR that are true replicas built with modern technology features. The TJ looks appealing but not a true 45, IMHO.
Thanks for the info 45. I don’t have an operating 45 amp at the moment, although one is in the works, and I don’t know anyone who has one either. It’s lonely in DHT land. Measured on an old Stark emissions only tester, I have several pairs that do match for emmissions, and the TJs that are perfectly matched in factory. Maybe I should sell off all the old 45s and buy a pair of EMLs?
Tubes that match on a tube tester might not match in the actual circuit you use them in (like at a different plate current, different plate voltage, and different grid bias).
I did not have problems with 45 tubes that had reasonable emission, even if they were not real well matched.
Be sure to use Separate bias, either separate self bias, or separate adjustable fixed bias.
With an SE amp using only one 45 used per channel, here are some potential differences:
1. How well each can balance out hum if they use AC filament power.
2. How much output power they produce
3. How much distortion at some power level
4. Grid current / gas
5. And important, but can be fixed with individual volume controls, or a balance control:
Gain in your circuit (relates to u, rp, and transconductance). u = gm * rp.
If 2 of those are good, the 3rd will be good too.
My SE stereo 45 amp was lots of fun, and good sounding. I used inexpensive 45 tubes I got at an Amateur Radio swap meet; not new old stock, just used.
I did not have problems with 45 tubes that had reasonable emission, even if they were not real well matched.
Be sure to use Separate bias, either separate self bias, or separate adjustable fixed bias.
With an SE amp using only one 45 used per channel, here are some potential differences:
1. How well each can balance out hum if they use AC filament power.
2. How much output power they produce
3. How much distortion at some power level
4. Grid current / gas
5. And important, but can be fixed with individual volume controls, or a balance control:
Gain in your circuit (relates to u, rp, and transconductance). u = gm * rp.
If 2 of those are good, the 3rd will be good too.
My SE stereo 45 amp was lots of fun, and good sounding. I used inexpensive 45 tubes I got at an Amateur Radio swap meet; not new old stock, just used.
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6A3sUMMER:
I’m glad to hear that you were successful using random 45s, and thanks for the info. I’ll be using Rod Coleman filament regulators, which are DC. The Hagerman Bugle is self biased., should that bias be also be adjustable? With respect to gain, having separate volume controls for each channel is easy enough, and lately I’ve found that due to the irregularities of my particular listening room, I wish every amp I owned had seperate volume controls. Actually a balance control would be great instead, as dual volume pots can be finicky and annoying.
I’m glad to hear that you were successful using random 45s, and thanks for the info. I’ll be using Rod Coleman filament regulators, which are DC. The Hagerman Bugle is self biased., should that bias be also be adjustable? With respect to gain, having separate volume controls for each channel is easy enough, and lately I’ve found that due to the irregularities of my particular listening room, I wish every amp I owned had seperate volume controls. Actually a balance control would be great instead, as dual volume pots can be finicky and annoying.
Easy to do. They're both so well known in the same circles.
By the way, re your first post , some experienced hands (old school?) consider differences from nominal of +/- 10% for individual tubes in a given circuit normal . . . even for new tubes. Adding up the tolerances for all the components in a circuit it can't be surprising that things will vary.
By the way, re your first post , some experienced hands (old school?) consider differences from nominal of +/- 10% for individual tubes in a given circuit normal . . . even for new tubes. Adding up the tolerances for all the components in a circuit it can't be surprising that things will vary.
tizman,
If you are going to use self bias, that will help you to do some reasonable matching.
If you can get a few 45 tubes you can just read the bias voltage of the two tubes and compare.
I do not know if there are amateur swap meets or antique radio swap meets in Toronto,
but if so, look for a few 45 tubes.
Sometimes even New Old Stock tubes can go gassy (I had one that went bad after not being used for (decades?), and re-activated for a few hours in one of my amps, then I turned it off. The next morning it was gassy and bad.
Take a couple that are close and then compare the gain of the two channels.
It does not have to be perfect, you can enjoy them if they are even close.
I would expect push pull might be a little more of a problem in regards to matching.
If you are going to use self bias, that will help you to do some reasonable matching.
If you can get a few 45 tubes you can just read the bias voltage of the two tubes and compare.
I do not know if there are amateur swap meets or antique radio swap meets in Toronto,
but if so, look for a few 45 tubes.
Sometimes even New Old Stock tubes can go gassy (I had one that went bad after not being used for (decades?), and re-activated for a few hours in one of my amps, then I turned it off. The next morning it was gassy and bad.
Take a couple that are close and then compare the gain of the two channels.
It does not have to be perfect, you can enjoy them if they are even close.
I would expect push pull might be a little more of a problem in regards to matching.
As I said with the 45's it's rather easy to find matched tubes even using a single point tester because they are low mu with moderate gm.
I have two quads one RCA and one Raytheon that I will soon bring to UK and curve-trace. They measure almost identical with each quad on the Hickk but different from one quad to the other. It's the Raytheon that shows unusually higher gm at 2.35-2.4 mA/V. The RCA's measure as expected from new tubes at 2.2-2.25 mA/V. Will see how matched they are overall but from what I have seen in amplifiers it's not an issue at all.
The EMLs have now gained reliability which was not the case some years ago. The KR I don't know but it looks like the original also aesthetically.
Those I could test from a friend of mine had more gm and gain. No a bad thing to have at all anyway. I didn't make that comment to dismiss them. The construction is 300-like or at least has been derived from that but it's just appearance.
About 2.6-2.7 mA/V and gain just over 4. Not a big difference but would be enough to classify it as not exact replica.
This was a few years ago. It was the one with black base and green printing. Things might have changed in the meantime.
Look here:
Sophia Electric's Home Page
If you go to the 45/n page there are curves and measured parameters. gm=3.2 mA/V and mu=4 at 250V/36mA. This also means lower plate resistance around 1260 R.
Maybe they can tell you more about typical values.
Sophia Electric's Home Page
If you go to the 45/n page there are curves and measured parameters. gm=3.2 mA/V and mu=4 at 250V/36mA. This also means lower plate resistance around 1260 R.
Maybe they can tell you more about typical values.
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