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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 19 Jun 2017, 07:57 
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Could i use kt88 instead of the el34 or 6v6 se amp?


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PostPosted: 19 Jun 2017, 09:01 
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A KT88 will draw slightly more heater current than a EL34 and much more than a 6V6.

6V6: 0.45A
EL34: 1.5A
KT88: 1.6A

EL34 and KT88 share the same pin layout. A 6V6 has a slightly different layout.

Hope this helps. Always check the datasheets and you can compare the differences for yourself.


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PostPosted: 19 Jun 2017, 14:09 
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Sonically, you couldn't ask for three more different output tubes.

If your gear can handle the change for reasons outlined above, you can have "three different amps" :D

Cheers!

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PostPosted: 19 Jun 2017, 17:55 
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Hi, My references indicate that unless you use the metal version (not the glass ones) of the 6V6 it shares the same pin out as the others. At various times pin one has been used for things. In the metal tubes it is the metal case and should be grounded. I understand that a few EL34s also used it as tie point. Good practice for all these tubes except the metal ones (6V6, 12V6, 6L6 etc) is to not attach anything to that socket pin. Unfortunately some manufacturers do use it...occasionally with rather exciting consequences. BTW older tubes that did not have a letter designation following the number (like 6V6) were metal cased octal (8 pin) tubes. I just got a batch of 12A6s to play with that are metal from the 60s. The Letter "G" after the number designates a glass tube. Additional letters (like 6V6GTA) indicate the shape of the glass..GT is the cylindrical shape, G is has a large bulge. The next letter (A, B etc) indicates later and often improved versions of the tube. A 6V6GTA will have slightly different specs from a 6V6GTB or 6V6GTC... and so on. EL and KT tubes do not follow this designation scheme.

Good listening
Bruce

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PostPosted: 19 Jun 2017, 20:14 
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gofar99 wrote:
Unfortunately some manufacturers do use it...occasionally with rather exciting consequences.

<--- see my avatar for said exciting consequences :D

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PostPosted: 20 Jun 2017, 11:23 
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maresaldeflota wrote:
Could i use kt88 instead of the el34 or 6v6 se amp?


Bias voltage is different on fixed bias, cathode bias resistor is different, unless the amp has CCS cathode.
Idle current is different and the output transformer needs to handle bias current. The 6v6 vs EL34 needs more iron in the transformer to prevent saturation.

As already pointed out heater current is different. that's about it really..
Its common practice in tube amps running EL34 to voltage double the bias supply to set the idle current for 6550 and other tubes in fixed bias. Be careful of anode voltage where the max voltage allowed can be very different between things like 6v6 and EL34.

If the amp is designed to run different tubes its easy to run>>EL34,5881,6550,6L6,6550,KT88, 807..yawn :D
however the 6v6 is in a different ball park...

I have never been able to live with 6550..Lumley used them.. what ever floats your boat.

Regards
M. Gregg

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PostPosted: 20 Jun 2017, 11:59 
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Quite true. I was only indicating the pins were the same. Plugging a 6V6 into a circuit set for one of the big guys will turn it into a light bulb. :hot: I even classify the 6L6GC, EL34, 6CA7s and KT77s as medium power tubes and the KT88, 6550 etc as large tubes. The dissipations between the mediums and big ones vary but are in the range of 30-50%. Using EL34s in a circuit for KT88s will probably make them into rather nice (but short lived) space heaters. :blush:

Good listening
Bruce

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PostPosted: 20 Jun 2017, 13:37 
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How about a kt 66 instead of el34?


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PostPosted: 20 Jun 2017, 16:28 
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gofar99 wrote:
Quite true. I was only indicating the pins were the same. Plugging a 6V6 into a circuit set for one of the big guys will turn it into a light bulb. :hot: I even classify the 6L6GC, EL34, 6CA7s and KT77s as medium power tubes and the KT88, 6550 etc as large tubes. The dissipations between the mediums and big ones vary but are in the range of 30-50%. Using EL34s in a circuit for KT88s will probably make them into rather nice (but short lived) space heaters. :blush:

Good listening
Bruce


I normally set cathode CCS for the top rating of the lowest power tube and just plug the others for fun running lower dissipation.
In this situation the 6v6 would be the limiting factor. I guess its possible to switch bias settings but there is always the chance some idiot (normally me) would leave it in the wrong setting after playing about 10 tube swaps in an hour.. :D :D

Most probably know,
In fixed bias EL34 you can get away with about -50v on the grid,
but to swap about between tubes you need probably double that -100 to be on the safe side.
In each case you still need to check and set the bias for each change.
Then you are down to automatic bias (servo)... :up: (or CCS)

Forgot to mention "mixed bias". :D

Regards
M. Gregg

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Last edited by M. Gregg on 20 Jun 2017, 17:01, edited 3 times in total.

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PostPosted: 20 Jun 2017, 16:43 
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maresaldeflota wrote:
How about a kt 66 instead of el34?


Why KT66? the most overpriced tube and not so easy to get.
Why not use 6L6 or 5881...I run EL34/KT66/6L6 etc in the same circuit.
There is always 807 but it has the horrible top cap.

I think you need to be specific about what you are trying to do, power, load function etc.
Or are you just trying to replace an EL34 with KT66 because everything is a trade off with tube loading.

You don't say why you wouldn't just build a KT88 amp..loads of circuits on the net.

Regards
M. Gregg

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