Biasing KT66 in stock ST70

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Biasing KT66 in stock ST70

Postby mugsy » Fri Jul 25, 2008 3:32 pm

Howdy folks!

Wondering if someone could point me in the right direction for biasing a quad of KT66s (I have both the Golden Dragon 'Retro' types (Shuguang I believe), and the Harma Watford Valves (rebadged Russian types) versions. (I also have six original GECs but they are all so badly matched I don't want to use those until I can get them properly tested, graded etc).

My built from scratch ST70 uses totally stock circuit values, except for the 80/40/30/20 Quad cap, and 100uF bias caps. When I try any KT66s the bias shoots up to past 1.56V VERY quickly, and this is with both bias pots at their minimum counter-clockwise settings. Does this point to something being wrong, as the Dynakit manual says KT66s should be compatible?

What values should I change the Bias resistors to so that I'm able to use either EL34s OR KT66s?

Many thanks.

- John
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Postby mesherm » Fri Jul 25, 2008 9:56 pm

When you say it shoots up past 1.56 volts how high does it go?
1.56 v = 50 ma per tube
1.72 v = 55 ma per tube
1.87 v = 60 ma per tube
2.03 v = 65 ma per tube

I don't think 60 ma or so on a KT66 will destroy it right away.
You may be very close to the bias voltage you need. A KT66 needs a bit more negative voltage than an EL34. If you have a 1/4 watt resistor between 10k and 20k temporarily solder it across the 10k (parallal with) resistor closest in the bias circuit to the bias diode. That should give you a few more negative volts to tame the idle current with and still work for EL34s. A little experimentation with resistor values should give you the right value that when paralleled with the 10k will allow you to bias both types of tube to 50 ma.
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Postby mugsy » Sat Jul 26, 2008 3:57 am

Thanks ever so much for your suggestion - it worked a treat Yellow_Light_Colorz_PDT_06

Soldered a 3 watt 18K resistor across the existing 10K, making a 6K4 value. Biasing the KT66s at 1.56 with no problem now.

Many thanks again - sounds fantastic with the Harma KT66s in place of the EH-EL34s (which were pretty darned good as well). More punch with the KT66s, and soundstage seems to be larger.


- John
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I musta been half asleep... KT77s not KT66s

Postby EWBrown » Sat Jul 26, 2008 5:47 pm

I used some of the J/J KT77s in an ST70, they needed only a modest bias setting "tweek" to hit 50 mA each tube. I'm using the VTA-70 board from Roy Mottram, which has individual power tube biasing, and a somewhat different voltage dividing scheme than the "stock" ST-70.

Yeah, I realize the original topic refers to KT66s, and not KT77s, but the "re-design" considerations for the fixed grid biasing should be pretty similar. FWIW the J/J KT66s are supposed to be pretty hefty "beasties" :o

If you are using an original Dynaco PA-060 power trannie, keep the tubes at 50 mA or less, but the Triode replacement and other new ones (Dynakiparts, etc, are heftier and you can crank up the current to 60-65 mA without having the local FD on standby Yellow_Light_Colorz_PDT_04

The limiting factor for B+ current will then be the rectifier tube, but SS rect can go the "full monty" and then the power trannie becomes the weak link...

I found that the JJ KT77s required a few volts less negative G1 bias voltage than EL34s do. The Chinese made KT66s will have somewhat different operating characteristics, YMMV, etc... I haevn't tried the SHuguang or other Chinese made KT66s or KT88s, only the EL34s and I wasn't too overly impressed with them... They're OK for testing and should be fine for power supply regulation, but I found thatr Svet and EH EL34s sounded better, YMMV, etc with the sino KT66s...

I haev some yet to be tried out Sussian KT-66s down in NC, waiting for me to get back to "fun" work... THe move comes first - and selling this NH dump, er, place, first...

FWIW, the early released J/J KT77s have "skinny" pins on their octal bases, and they will not make good reliable contact in most tube sockets. J/J corrected this issue after the first few lots went out.

/ed B
Last edited by EWBrown on Tue Jul 29, 2008 1:09 pm, edited 2 times in total.
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Postby mugsy » Sun Jul 27, 2008 1:40 am

I have the VTA board as well, but I like the sound (hum notwithstanding) of the stock ST70 so much I think I'll stick with it for now. Can't understand why some people diss the stock ST70 sound. Built from new parts (except transformers, in my case) it sounds every bit as dynamic as modern gear, damned more musical as well.

Funnily enough I **think** I prefer the sound of the EH-EL34s in my ST70 - the sound just seems to gel. Beautiful top end extension and silky smooth. Nice bass kick too. Lovely midrange presence.

Still the VTA board would come in very handy for being able to use my wildly mismatched set of GEC KT66s... hmmm what to do what to do! Yellow_Light_Colorz_PDT_09
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Postby Bob01605 » Sun Jul 27, 2008 7:26 am

mugsy wrote:I have the VTA board as well, but I like the sound (hum notwithstanding) of the stock ST70 so much I think I'll stick with it for now. Can't understand why some people diss the stock ST70 sound. Built from new parts (except transformers, in my case) it sounds every bit as dynamic as modern gear, damned more musical as well.

Funnily enough I **think** I prefer the sound of the EH-EL34s in my ST70 - the sound just seems to gel. Beautiful top end extension and silky smooth. Nice bass kick too. Lovely midrange presence.

Still the VTA board would come in very handy for being able to use my wildly mismatched set of GEC KT66s... hmmm what to do what to do! Yellow_Light_Colorz_PDT_09


I don't really agree with you on the ST-70 sound with a stock or replacement stock driver board. In many comparisons with the VTA, DIYtube and other upgrade driver board, the stock driver board circuit comes up a little bit short.

This is not to say that the stock ST-70 driver board sounds "bad". With a stock driver board, the sound is mellow, with loose bass and rolled off on the top end. It is a non fatiguing and non irritating sound which IMHO isn't as ACCURATE as many of the upgrade boards. One problem is the pentode/triode 7199 tube which is OK soundwise but not as good as modern all triode designs.

Dynaco got away with this driver board circuit and the weak power supply because the main signal source of the day, vinyl records, has both limited high frequency response and limited dynamic range which didn't tax the weak power supply that much.

I do agree with John above on the use of EH EL34's in a stock ST-70. They seem to have a little extra high frequency energy which serves to offset the slightly lowered frequency response of the stock driver board circuitry.

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Postby burnedfingers » Sun Jul 27, 2008 8:12 am

I thought the stock board was marginal at best. If your not used to listening to tighter bass you might except it without question. I also think the VTA board is a good sounding board with the nice feature of individual tube bias. I experienced some hum with the board that I didn't care for.
I don't agree that triodes are best. My personal favorite for a driver board is the Mapletree board with the low noise 5693 input tube and the 5691 phase splitter.
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Postby Bob01605 » Sun Jul 27, 2008 11:30 am

burnedfingers wrote:I thought the stock board was marginal at best. If your not used to listening to tighter bass you might except it without question. I also think the VTA board is a good sounding board with the nice feature of individual tube bias. I experienced some hum with the board that I didn't care for.
I don't agree that triodes are best. My personal favorite for a driver board is the Mapletree board with the low noise 5693 input tube and the 5691 phase splitter.


I have used over 75 VTA driver boards in kits and wired amps in the past two years. Not ONE VTA driver board had any problem with "hum". My suggestion is that the hum you experienced from your installation of the VTA driver board was related to some other issue on your amp and/or improper contruction or installation of the board.

I have never heard the Mapletree board in an ST-70 so I cannot comment on its sound. It is a little pricy at $250 and I don't believe he offers the board in "kit" form. It also uses the old bias system with one bias pot for each channel instead of one bias pot for each output tube as on the VTA driver board.

Your comments about the 5693 and 5691 would lead one to believe that both tubes on that board are pentodes. The 5693 initial voltage amplifier is a pentode but the 5691 phase splitter is a dual triode tube.

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Postby K-MAN » Sun Jul 27, 2008 2:53 pm

How do those big tubes sound compared to the stock EL34? I love the look of the amp with something like KT66 or KT88's on them, I'm seriously considering building my own but I know they need a large power transformer so I guess I would build it from scratch with new parts.
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Postby burnedfingers » Sun Jul 27, 2008 3:19 pm

Bob,

Quote:

My suggestion is that the hum you experienced from your installation of the VTA driver board was related to some other issue on your amp and/or improper contruction or installation of the board.

My board construction was to the letter in both detail and operation on the VTA board. Since I don't have that particular Dynaco anymore I do not have the ability to see if it had a ground loop problem. I believe there was a ground trace around the parameter of the board but to be honest I cannot remember. You would be able to answer the question as to there being a ground trace around the mounting hole areas on the board and if the usage of metal screws to fasten the board to the chassis did contribute to a ground loop condition.

I did experience a hum problem with the Blackburn board that I solved by floating the board from the chassis and using plastic non-conductive hardware to mount the board.

Your correct in stating the Mapletree board does not have the ability to adjust individual tube bias but then again that is something that just about anyone could do in 15 minutes with an electric drill to make two extra holes in the chassis and the ability to wire in two additional bias pots.

Quote:

Your comments about the 5693 and 5691 would lead one to believe that both tubes on that board are pentodes. The 5693 initial voltage amplifier is a pentode but the 5691 phase splitter is a dual triode tube.


Bob, I feel that anyone with the ability to read or be read to could easily understand the need to look up the tube numbers that I gave if they didn't know the tubes and having done so would easily come to the conclusion that I didn't try to lead anyone into thinking that both tubes are pentodes. The 5693 is clearly listed in any tube manual as a pentode and the 5691 is clearly listed as a dual triode. To make things even clearer I could mention 6sj7 and 6sl7. To make it so that you can clearly understand I will say 6sj7/5693 sharp cutoff pentode and 6sl7/5691 is a dual triode.

Your statement that the triode is better is a matter of your opinion and certainly not shared by me. Having had the VTA board the opinion that I arrived at was I found it to be irritating and shrill with the JJ tubes as purchased from Roy and not much better with low noise numbered tubes that I will not mention so I don't lead anyone a stray here. I can understand your need here Bob to be defensive since you are in the business of pushing your line of the Dynaco product with Roy's board.
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Postby Bob01605 » Sun Jul 27, 2008 6:07 pm

Quote
"My board construction was to the letter in both detail and operation on the VTA board. Since I don't have that particular Dynaco anymore I do not have the ability to see if it had a ground loop problem. I believe there was a ground trace around the parameter of the board but to be honest I cannot remember. You would be able to answer the question as to there being a ground trace around the mounting hole areas on the board and if the usage of metal screws to fasten the board to the chassis did contribute to a ground loop condition."

There is a ground trace around the perimeter of the board and the screw holes are included in the ground trace. This situation has NEVER caused a ground loop/hum situation in any amp that I have assembled with the VTA board (25- 30 amps). I would suggest that your amp did have some OTHER type of ground loop issue since if it was installed "to the letter in both detail and operation" as you mention it will cause no "ground loops" and/or hum.

Quote
"Your statement that the triode is better is a matter of your opinion and certainly not shared by me. Having had the VTA board the opinion that I arrived at was I found it to be irritating and shrill with the JJ tubes as purchased from Roy and not much better with low noise numbered tubes that I will not mention so I don't lead anyone a stray here. I can understand your need here Bob to be defensive since you are in the business of pushing your line of the Dynaco product with Roy's board."

Roy's board was designed in the 1980's and Roy has continued to refine the board over the years. The fact that it sounds good and has proven itself to be dead reliable is the reason I use it. You may have had a bad experience with it for reasons not clear to me but your opinions are not echoed by the literally thousands of customers that have purchased the board from Roy over the past 20+ years. Of the 75+ boards that have been shipped by me in the past 2 1/2 years there have been ZERO board component failures and NO reports of any 'hum" asociated with the installation of the board when installed "to the letter in both detail and operation" as per Roy's instructions.

Bob
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back to topic - KT66s

Postby tubes4hifi » Sun Jul 27, 2008 6:31 pm

looks like mugsy got his original question answered, yes, KT66s work fine in ST70s with just a change in the bias.
Everyone has a different taste in tubes (and food, and women, and . . . . )
Some like KT88s, some EL34s, some KT66s or KT77s
some like triodes, some like pentodes, some like solid state silicon (OMG!)
I like to hear good natural sound, and to each his own
nice to know alot of people have tried various amps, drivers, and tubes.
Enjoy the music !
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Postby mugsy » Wed Jul 30, 2008 9:27 am

Exactly... each to their own :)

I have to say I don't find the stock Dynaco circuit 'muddy sounding', vague, bass light or any of the other criticisms levelled at it. Perhaps this comes from using modern components, hand matched. If I could solve this blessed hum problem I don't think I'd need to upgrade the driver board at all.

My current amplifier reference is the Greg Ball GB150D, but my Dynaco is very enjoyable to listen to and in some respects makes music more 'fun', even though it may not be accurate, strictly speaking. Anyways, when I get tired of one I can always switch to the other, and back again ;)

- John

P.S. How can I tell if it's the quad cap causing hum? Heard many reports of these being faulty and unreliable...

P.P.S. Burnedfingers: "I thought the stock board was marginal at best" - you seemed to like it according to your posts on www.diyaudio.com (once you'd replaced the old parts on the driver board)...?
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Postby mesherm » Wed Jul 30, 2008 1:51 pm

P.S. How can I tell if it's the quad cap causing hum? Heard many reports of these being faulty and unreliable...


If you have a 20 to 40 uf 500v cap around temporarily tack solder it in parallel with each section of the quad cap starting with the one at the 22k resistor, then the 6.8k and so on. Most likey its one of those two cap sections though. If the hum goes away you have found the bad section.
Keep in mind that you are dealing with HIGH VOLTAGES so take appropriate precautions.
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Postby burnedfingers » Thu Jul 31, 2008 5:16 am

Quote:

P.P.S. Burnedfingers: "I thought the stock board was marginal at best" - you seemed to like it according to your posts on www.diyaudio.com (once you'd replaced the old parts on the driver board)...?

Do you have a particular post in mind? There are probably one or two comments that I mention that changing parts made an improvement. If I was really impressed with the performance of the stock board then why would I change it? If you have something you would like to discuss please do email me and I would be glad to comment.

To those that find the bass response of the stock Dynaco enjoyable ... maybe you haven't experienced really good audio gear in your short term of audio. Bass is supposed to be tight, fast, and punchy. Unfortunately there are those that consider the boom boom of car stereo to be bass. Unfortunately because I have to play with high powered systems I have little tolerance for the sound of junk and I am highly critical of everything I listen too and therefore try to make things as good as possible.

In checking your cap..in some instances it is best to substitute the cap in question with a suitable loaner. If the cap in question is real leaky you may not see a performance improvement or any lowering of the hum. Do not exceed the value of your first cap because you may experience a flash over in the 5AR4 or damage it. You may also want to change out the caps in the bias circuit as it can also contribute to hum in the amplifier.

A nice modification for those interested in using other tubes would be to wire a switch and resistor in parallel with the 10K resistor in the bias circuit or better yet would be a -on-center off-on- switch. This would allow for two selections of resistance to be paralleled with the stock 10K resistor. Note only one resistance value at a time in each direction and a STOCK setting in the center detent. THis would allow for example KT 66's in one switch position, the stock EL34 position, and another selection such as 6BG6GA's in the last selection. Since the bias voltage and current are so low there would be no problem with the switch handling it.

In my opinion the best modification to ANY Dynaco Stereo 70 is the addition of individual bias controls for each output tube. There are some that would argue that it isn't necessary but as we know tubes do change and even the best matched tubes do NOT stay matched for very long.
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