KT77 application in Stax amps

[mwl168]

Not sure which forum is the most appropriate place to post this topic. since it's technical in nature I chose this DIY forum.

My question is - which Stax amps can one safely use the KT77 in place of the EL34 output tubes and why.

Judging by the KT77 spec as stated on the datasheet, it should be compatible to the EL34 in terms of plate voltage and dissipation, etc.. But it has been stated by many that the KT77 should not be used in the BHSE. Can someone explain the technical reasons behind it? Also, what about using the KT77 in the Grounded Grid, Megatron and the DIY T2 and why.

For me, it's both a learning exercise as well as a practical question - I've used the true beam tetrode 6CA7 in my Grounded Grid and Blue Hawaii (DIY) to good effects and wonder how the KT77 may fair in these applications. 

Thanks in advance for sharing the knowledge and thoughts.

 

Edited December 27, 2019 by mwl168

[joehpj]

KT77 has different internal configuration. Take JJ's spec sheet for example, 6CA7 pin1 was connected to g3 while KT77 has pin1 NC and g3 connected to cathode internally.  It won't be a problem in some versions of the Kevin's layout which G3 and cathode was connected on the PCB. But in a certain version of GG's layout, which G3 was linked to anode. It will be a disaster if KT77 was plugged in. I didn't checked the BH's board layout. So if your BH/GG has former board layout, it should be perfectly fine. 

[jose]

On 5/13/2018 at 4:31 AM, Laowei said:

EL34, 6CA7 and KT77 all have the same functional pinout, so don’t know why adapters would be needed.

EL34s and small bottle 6CA7s are pentodes with suppressor G3 brought out on pin 1.

Big bottle 6CA7s and KT77s are beam power tubes. NOS and Russian reissue tubes are tetrodes and have the beam forming plates internally tied to the cathode. JJ 6CA7s and KT77s are beam power pentodes which the beam forming plates are brought out on pin 1 for external connection. Depending on the circuit these JJ tubes may cause problems if the beam forming plates are not connected to cathode potential. 

Justin at Headamp explicitly says NOT to use the KT77 in his BHSE amps. SO DON’T USE THEM IN HIS AMPS. 

I’m currently using a quad Gold Lion KT77s in my GG, and liking them a lot. The opto-servo in the GG might be the reason they work here (so far.....). Does not necessarily mean they will work as a EL34 replacement in all cases.

 

[mwl168]

Thanks to joehpj and Jose. That’s helpful.

I have additional questions. Both KT77 and true 6CA7 (often referred to as the “big bottle” or “ fat boy” 6CA7) are beam tetrode while EL34 is a pentode hence the different construction as explained above. Yet, only KT77 tubes are reported to cause problems when used in the Blue Hawaii (and maybe the DIY T2). Why? 

I have used a quad of big bottle EH 6CA7 on my Blue Hawaii without issue and they sound really good! 

[jose]

As everyone knows, I have been using "fat boy" 6CA7 (Electro-Harmonix) since I set up the GG and I also used them in my Megatron for a while and I had no problems. In fact, I have several replacement quads.

I haven't used them in my BH but I agree with you Michael that I think these fat ass girls ? "sound" better than the EL34.

* From what I've read, the Electro Harmonix 6CA7s are true beam tetrode. 

 

Edited December 29, 2019 by jose
orthography

[Laowei]

The trouble is with the non-standard pin out of JJ beam power “pentode” tubes (6CA7 and KT77) as seen in the DIY T2 circuit .
 

In the DIY T2 (as well as the original T2), G3 (pin1) on the PCB are connected to the plate (B+). Using the big bottle JJ 6CA7 with its G3 brought out to pin 1,  effectively makes the beam forming plates the anode. They were not designed to take the full plate load current. On all other Kevin Gilmore amps (except the all tube Grounded Grid amps which no one has built) connect G3 to the cathode (G1) on the PCBs.

Using true pentode EL34/6CA7 or G3 internally cathode connected beam power tetrodes work fine here. Using JJ 6CA7s or JJ KT77s are the problem. In the DIY T2, true pentode= OK, JJ Beam power = problem, all other Beam power = OK. It’s easy to visually verify if pin 1 on the JJs  is internally connected or not. 
 

Justin has his own customer histories that makes him advise not to use KT77s in his amps. So don’t do it there!  

 

Edited December 31, 2019 by Laowei

[mwl168]

Thanks 老外！I’ve read your posts on this subject and was hoping you would chime in when I started this thread. 

Care to fathom a guess/theory why Justin advised against using KT77 on his BH amps? 

I recalled Kevin tried KT77 in his BH way back in 2007 but later in a PM said that KT77 is unstable at 800V +. 

新年快樂！

[Laowei]

I haven’t heard/read the actual reason so can’t speak to Justin’s stated policy of not using KT77s in his amps.

Whether a design flaw in those particular tubes substituting for EL34s or his customers’ experience with a bad batch or two are possibilities, but only unfounded speculation.

Just don’t put any KT77s in his amps.

[guzziguy]

23 hours ago, mwl168 said:

Care to fathom a guess/theory why Justin advised against using KT77 on his BH amps?

Why don't you just PM him with the question?

[JoaMat]

On ‎12‎/‎31‎/‎2019 at 6:12 PM, Laowei said:

On all other Kevin Gilmore amps (except the all tube Grounded Grid amps which no one has built) connect G3 to the cathode (G1) on the PCBs.

Not sure which one of all Kevin’s designs the “all tube Grounded Grid” is. But in mid-2017 both members congo5 and MattN have built what seems to be something similar to Kevin’s KGSSHV-TUBE- SANDWICH – ECC88 input and EL34 output tubes.

Ref. https://www.head-case.org/forums/topic/12949-stax-t8000-clone-well-sorta/

[jose]

I think that Michael and I talked about that project at that time and decided to put our attention to the T2 mini.

I don't remember how the two Sandwich prototypes ended. Maybe Congo or Matt can tell us something.

Edited January 3, 2020 by jose

[Laowei]

That’s the one I was thinking of JoaMat.  The “Sandwich Amp” is only other electrostatic tube amp besides the DIY T2, that ties G3 to the Plate on the PCB. All others tie G3 to Cathode. 

[mwl168]

The JJ KT77 datasheet states the tube as an "A. F. beam pentode" and shows a drawing of internal construction that looks like a pentode with the third grid internally connected to the cathode. But the base connection diagram shows pin 1 and 6 as not connected. 

Both the current and old production Genalex KT77 datasheets state the tube as a "beam tetrode" and the base connection diagram shows pin 1 and 6 as not connected just like the JJ KT77. There is no drawing of internal structure on either datasheet.

Every thing I have read so far on the web refer to KT77 as a tetrode constructed similar to a 6L6. 

The JJ 6CA7 datasheet on the other hand has internal structure drawing of a pentode with the third grid connected to pin 1 and states the tube as an "A. F. output pentode". Yet the photo of the actual tube is a "fat boy" type big bottle similar to the Sylvania, GE and EH 6CA7 which are all true tetrode.

You got to love this tube business...  

 

[JoaMat]

If interested in the story of good old tubes, I recommend Guide to the EL34/6CA7 by Pasquale Russo. Also some information on the KT77. 365 pages of interesting stuff.

[JoaMat]

Four Gold Lion KT77 in a mini T2. G3 connected to plate on PCB.

It rocks Beautiful!

[Laowei]

Those reissue GL KT77s sounded sweet in my GG too.