KT88 dissipation question

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KT88 dissipation question

Postby muypogi » Wed Sep 09, 2009 9:21 am

I'm looking to change my 4 x KT88 tube complement with 4 x 6550 Tung Sol reissues. I'd like to know if the Tung Sol reissues are drop in replacements for my AMX Forte amp, which is 70wpc I think.

The guy who I'm dealing with in the states (Jim McShane, nice tube guy who refurbs HK Citation amps) says that the Tung Sol reissues were modeled after a 6550 variant which has only 35 watts dissipation, vs 46 watt dissipation for the 6550A and 6550C models, both of which can be drop in KT88 replacements.

Any suggestions?
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Re: KT88 dissipation question

Postby alexg » Thu Sep 10, 2009 9:02 am

If I were you, I would ask Andrew of AMX how hot he biased his amp and if he would recommend using the Tung Sol 6550 with lower max plate dissipation.

I have used the Tung Sol 6550 reissues on my KT88 SE amp, but my amp is biased very conservatively. The Tung Sol 6550s are quite nice sounding, quite similar to the GE 6550.
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Re: KT88 dissipation question

Postby muypogi » Thu Sep 10, 2009 5:37 pm

alexg wrote:If I were you, I would ask Andrew of AMX how hot he biased his amp and if he would recommend using the Tung Sol 6550 with lower max plate dissipation.

I have used the Tung Sol 6550 reissues on my KT88 SE amp, but my amp is biased very conservatively. The Tung Sol 6550s are quite nice sounding, quite similar to the GE 6550.


I already asked if I could change to 6550s given their lower rating, pero no response yet. Also posted my question on the AMX thread in PinoyDVD. . . :worried:
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Re: KT88 dissipation question

Postby muypogi » Sat Sep 12, 2009 6:12 am

Just to update, I decided to go with a quad of SED Winged C 6550C tubes. Reviews on the internet show it's the best value for sound quality and durability. Also, my tube seller recommended it for bass, next to the more expensive Gold Lion reissue. Boss Andrew noted that the Tung Sol may not like being in a KT88 amp.

Hoping this solves my bass problem. . . :emo:
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Re: KT88 dissipation question

Postby qguy » Sat Sep 12, 2009 7:14 am

I use a New production Tung-sol 6550 in my Cayin A-70T and the default tube is EH Kt88, mataba ang bass. No problems whatsoever...

muypogi wrote:Just to update, I decided to go with a quad of SED Winged C 6550C tubes. Reviews on the internet show it's the best value for sound quality and durability. Also, my tube seller recommended it for bass, next to the more expensive Gold Lion reissue. Boss Andrew noted that the Tung Sol may not like being in a KT88 amp.

Hoping this solves my bass problem. . . :emo:
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Re: KT88 dissipation question

Postby alexg » Sun Sep 13, 2009 11:11 pm

muypogi wrote:....Hoping this solves my bass problem. . . :emo:


KT88 and variants are supposed to have very good bass, is your AMX amp lacking in bass?

Maybe your input/driver tubes is the problem.
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Re: KT88 dissipation question

Postby muypogi » Mon Sep 14, 2009 3:26 am

alexg wrote:
muypogi wrote:....Hoping this solves my bass problem. . . :emo:


KT88 and variants are supposed to have very good bass, is your AMX amp lacking in bass?

Maybe your input/driver tubes is the problem.


All this happened when I removed my amp's tone controls. Andrew said the thin bass may have been due to: 1) tone controls adding just a bit of bass even in the 12 o clock position and 2) my power tubes are growing old, since they've been there for 2 years and have seen some fairly good use (the amp is for audio and video playback).

I don't think it's the driver tubes since they were changed to Sovtek 6N1p and Mullard 12AT7 equivalents just before I sent the amp for repair.

In any case, removing the tone controls made the amp's highs and mids noticeably clearer, but at the expense of bass. . .
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Re: KT88 dissipation question

Postby arnoldc » Mon Sep 14, 2009 8:46 am

Without the tone control, now you know how it sounds like. Have it re-designed, even change the output transformer, or even the entire power supply.
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Re: KT88 dissipation question

Postby alexg » Mon Sep 14, 2009 11:18 am

arnoldc wrote:Without the tone control, now you know how it sounds like. Have it re-designed, even change the output transformer, or even the entire power supply.


6n1p and 12at7 are good drivers, and should not sound lacking in bass, in fact, some 12at7 will cause excessive bass with the wrong operating points.

Arnoldc has a very valid point, the design, the transformers and the PSU might be the cause.

I don't think replacing the KT88 with 6550 will solve the problem.
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Re: KT88 dissipation question

Postby Agent007 » Mon Sep 14, 2009 12:40 pm

muypogi wrote:
All this happened when I removed my amp's tone controls. Andrew said the thin bass may have been due to: 1) tone controls adding just a bit of bass even in the 12 o clock position and 2) my power tubes are growing old, since they've been there for 2 years and have seen some fairly good use (the amp is for audio and video playback).

I don't think it's the driver tubes since they were changed to Sovtek 6N1p and Mullard 12AT7 equivalents just before I sent the amp for repair.

In any case, removing the tone controls made the amp's highs and mids noticeably clearer, but at the expense of bass. . .

:hai: Removing the tone control is a good idea but it takes a lot of work to tweak ur input stage using the same tube to bring the bass back...remember tone control is an equalizing network for compensation between ur input and driver stages, even positioned at 12 O clock it will still give u a profound effect, so without it ur input stage signal output is lean in bass... so if you are technically skilled to do the tweaking I suggest u better start there before anything else, as suggested earlier by arnoldc and alexg, to keep ur approached economical and prudent. If you are not capable of doing it leave it to the experts give them some work to do... :lol: :lol: ;)

I've done a lot of work on this particular subject to amplifiers of any brand whether solid state or vacuum-tube - it is my simplest attack and the next is to look at ur power supply stage - stiff it up... :!: Good luck!
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Re: KT88 dissipation question

Postby muypogi » Mon Sep 14, 2009 4:41 pm

arnoldc wrote:Without the tone control, now you know how it sounds like. Have it re-designed, even change the output transformer, or even the entire power supply.


When I sent it in, I actually asked if it was feasible to beef up the power supply/transformers. The answer was that it was an unnecessary expense. I had wanted to see if I could beef up the transformers at least. I insisted though that the rectifier be changed, which he obliged with fast-acting diodes.

I am aware that the current sound might be the actual sound without the tone controls. I like the improvement in the high and mids. What I'm missing is the low-end, which nicely rounded the sound of the amp.

In any case, my tubes needed replacement anyway, so let me see if it will solve my problem. If not, then I may have to ask the tube gurus here to take a peek at the amp and see what can be done. Of course, any recommendation will need to coincide with my cash flow. . . my current cash flow is being drained by some needed repairs on my van. . . :(
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Re: KT88 dissipation question

Postby tony » Tue Sep 15, 2009 3:48 am

a lot depends on how you listen, do you play loud or soft?

our ears percieve loudness in a logarithmic manner...you will notice that at low levels, music may not be as enjoyable as when the volume is at say 11'oclock...

this is the reason for tone controls......some even put loudness circuits to accentuate the lows and highs at low volume levels...

a simple tweak you can do is to up the power supply capacity, that is if your rectifiers can handle the inrush currents that added capacitance in the power supply brings. ;)
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Re: KT88 dissipation question

Postby muypogi » Tue Sep 15, 2009 5:07 am

tony wrote:a lot depends on how you listen, do you play loud or soft?

our ears percieve loudness in a logarithmic manner...you will notice that at low levels, music may not be as enjoyable as when the volume is at say 11'oclock...

this is the reason for tone controls......some even put loudness circuits to accentuate the lows and highs at low volume levels...

a simple tweak you can do is to up the power supply capacity, that is if your rectifiers can handle the inrush currents that added capacitance in the power supply brings. ;)


I play between 9-11 o clock usually, since any higher and the neighbors will likely complain. . . :lol:

I removed the tone controls since I found out that after 2 years of ownership, I barely touched them beyond the 12 o clock position. I had not known that even when flat tone controls could add color to your amp's sound. I had a NAD amp before and the best sound I got really was when you either put the tine controls at the 12 o clock position or when I pressed the tone defeat button - both methods produced roughly the same effect, as far as I can remember.

How would you propose to increase the power supply capacity? Bigger transformers? How much would they cost nowadays? I had asked if moving to toroids would be beneficial, and I was told that the conventional transformers would be the best, as toroids would offer no sonic advantages and are prone o current surges.
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Re: KT88 dissipation question

Postby qguy » Tue Sep 15, 2009 8:43 am

hmmm....

Your amp seems to have gone through several changes / fixes and may require more to get it to sound right to your ears..
Might be cheaper in the long run to get a new amp that fit your needs.
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Re: KT88 dissipation question

Postby alexg » Tue Sep 15, 2009 8:52 am

muypogi wrote:
tony wrote:a lot depends on how you listen, do you play loud or soft?

our ears percieve loudness in a logarithmic manner...you will notice that at low levels, music may not be as enjoyable as when the volume is at say 11'oclock...

this is the reason for tone controls......some even put loudness circuits to accentuate the lows and highs at low volume levels...

a simple tweak you can do is to up the power supply capacity, that is if your rectifiers can handle the inrush currents that added capacitance in the power supply brings. ;)


I play between 9-11 o clock usually, since any higher and the neighbors will likely complain. . . :lol:

I removed the tone controls since I found out that after 2 years of ownership, I barely touched them beyond the 12 o clock position. I had not known that even when flat tone controls could add color to your amp's sound. I had a NAD amp before and the best sound I got really was when you either put the tine controls at the 12 o clock position or when I pressed the tone defeat button - both methods produced roughly the same effect, as far as I can remember.

How would you propose to increase the power supply capacity? Bigger transformers? How much would they cost nowadays? I had asked if moving to toroids would be beneficial, and I was told that the conventional transformers would be the best, as toroids would offer no sonic advantages and are prone o current surges.


You can up PSU capacitance by adding capacitors to it, or replacing capacitors with higher capacitance once. Audio Amplified carries some nice mundorf PSU caps that will not break your bank.

If I own your amp, I will check the operating points of the driver tubes. I found that on some driver tubes, higher plate voltage with higher current results in a bit of a sibilance, putting good quality bypass capacitor on the cathode of the drivers and power tubes seems to increase bass too.

I have seen some locally made amps with transformers constructed out of cores that are not suited for audio, and usually, these irons gets so hot that you can fry egg on it. :rofl:

Like most tube gurus say, the quality of tubed equipment relies heavily on the quality of the transformers... :!:
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Re: KT88 dissipation question

Postby muypogi » Tue Sep 15, 2009 9:01 am

qguy wrote:hmmm....

Your amp seems to have gone through several changes / fixes and may require more to get it to sound right to your ears..
Might be cheaper in the long run to get a new amp that fit your needs.


I won't dispute that, but getting a new amp will likely break the bank and will banish me from the conjugal bedroom for the rest of the year. . . :rofl:

I actually wanted a Cayin A55T or the SP10A, but those amps were (and still are ) way above my reach. So I will have to make do with what I have. Here's hoping the new tubes can tide me over while I save up for a possible rework of the amp. :hai:
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Re: KT88 dissipation question

Postby muypogi » Tue Sep 15, 2009 9:04 am

alexg wrote:
muypogi wrote:
tony wrote:a lot depends on how you listen, do you play loud or soft?

our ears percieve loudness in a logarithmic manner...you will notice that at low levels, music may not be as enjoyable as when the volume is at say 11'oclock...

this is the reason for tone controls......some even put loudness circuits to accentuate the lows and highs at low volume levels...

a simple tweak you can do is to up the power supply capacity, that is if your rectifiers can handle the inrush currents that added capacitance in the power supply brings. ;)


I play between 9-11 o clock usually, since any higher and the neighbors will likely complain. . . :lol:

I removed the tone controls since I found out that after 2 years of ownership, I barely touched them beyond the 12 o clock position. I had not known that even when flat tone controls could add color to your amp's sound. I had a NAD amp before and the best sound I got really was when you either put the tine controls at the 12 o clock position or when I pressed the tone defeat button - both methods produced roughly the same effect, as far as I can remember.

How would you propose to increase the power supply capacity? Bigger transformers? How much would they cost nowadays? I had asked if moving to toroids would be beneficial, and I was told that the conventional transformers would be the best, as toroids would offer no sonic advantages and are prone o current surges.


You can up PSU capacitance by adding capacitors to it, or replacing capacitors with higher capacitance once. Audio Amplified carries some nice mundorf PSU caps that will not break your bank.

If I own your amp, I will check the operating points of the driver tubes. I found that on some driver tubes, higher plate voltage with higher current results in a bit of a sibilance, putting good quality bypass capacitor on the cathode of the drivers and power tubes seems to increase bass too.

I have seen some locally made amps with transformers constructed out of cores that are not suited for audio, and usually, these irons gets so hot that you can fry egg on it. :rofl:

Like most tube gurus say, the quality of tubed equipment relies heavily on the quality of the transformers... :!:



Am not the tube DIY kind of guy, so I hope either you one of the tube gurus can take a look and see if there's any way to add bass. :clap:
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Re: KT88 dissipation question

Postby tony » Tue Sep 15, 2009 9:20 am

How would you propose to increase the power supply capacity?


i really meant capacitors when i said psu capacity.....just like what alex said....

although getting a bigger power transformer with lower dc resistances on windings is another way to do it, i won't reccomend it if you can not dipsose of the original power transformer to recoup some of your cost.

for new designs i would go for separate filment and plate traffos, this way, you can have more freedom to tweak the plate traffo should one find it needs upgrading....this is the plan i favor now.... :)
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Re: KT88 dissipation question

Postby muypogi » Wed Sep 16, 2009 9:16 am

Thanks for all the inputs mga sirs. I'm currently awaiting my tubes, which should arrive either this weekend or next week, depending on how fast Johnny Air can send it from their NY office. If those 6550s solve my problem, then I'm set. :wasntme:

Otherwise, maybe I can entice the tube gurus to take a peek at the amp and see how I can improve it. Have no DIY skills in tube amps so I wouldn't even know how to open it up so I can at least take pics of the circuits. . . :$
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Re: KT88 dissipation question

Postby tony » Fri Sep 18, 2015 7:35 am

arnoldc wrote:Without the tone control, now you know how it sounds like. Have it re-designed, even change the output transformer, or even the entire power supply.


very good advice for tube amps...it is the OPT's and not the tubes per se...
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