Dennis Had Inspire Amps

[robert_kc]

My favorite thus far: 6L6GC / 5V4G / 6DJ8.

Specific favorites:

• Output: Groove Tube Gold Series GT6L6GE Performance Rating 5 (#2: Russian military 6P3S-E, #3 Tung-Sol 6L6G, #4: KT150)
• Rectifier: Sylvania JAN 5V4G (#2 : various 5V4G & 5V4GA, #3 Mullard 5AR4)
• Input: Amperex 6DJ8 (Holland)

Here’s the output tubes that I’ve tried:

Zenith 6V6G (vintage)
Sylvania 6L6GA (vintage)
Sylvania 7408 (vintage, 6V6 variant)
Mullard EL34
Bugle Boy EL34 (vintage)
Russian 6P3S-E (Russian military surplus 1980s 6L6GC equivalent)
Groove Tube Gold Series GT6L6GE (6L6GC)
McIntosh branded GE 6L6GC (vintage)
GE 6L6GC (vintage)
Tung-Sol 7581A (6L6GC variant)
Tung-Sol 6L6G (6L6GC in a big bottle)
Gold Lion KT88
Tung-Sol KT150

Rectifiers that I’ve tried: 5Y3, 5U4, 5U4GB, 5V4, 5V4GA, 5AR4 (multiple manufacturers of each).

Front-end (9 pin miniature socket) tubes that I’ve tried: 6CG7, 6N1P-EB, 6DJ8, 6N2P-EV, E88CC.

I probably should have bought “socket savers” – because according to my notes I’ve conducted 90 tests. I think I’m done tube-rolling for a while.

The good news, I’m very pleased with sound quality.

My system where the Inspire amp is installed:

• Inspire Inspire “Fire Bottle” SE Stereo Tube Amplifier HO (originally equipped with KT88)
• CD/SACD player: Oppo DV-980H (provides crossover to subwoofer)
• Speakers: Klipsch RF-7II
• Subwoofer: Klipsch R-115SW

Music: Classical, opera, big band

[Bombadil]

Robert_KC, your Inspire amp uses 6DJ8 as the driver tube? My Inspire SE EL84 does, but I hadn't seen any of his KT88 amps use that tube. Wouldn't mind it if mine did, as I have several very nice Amperex 6DJ8-types.

[robert_kc]

Originally Posted by Bombadil

Robert_KC, your Inspire amp uses 6DJ8 as the driver tube? My Inspire SE EL84 does, but I hadn't seen any of his KT88 amps use that tube. Wouldn't mind it if mine did, as I have several very nice Amperex 6DJ8-types.

My Inspire amp originally shipped with:

• KT-88 GENALEX OUTPUT TUBES
• 6CG7 NOS INPUT DRIVER
• 5U4 Shuguang RECTIFIER TUBE

Following is one of Dennis’ emails about tube compatibility in my amp:

GT6L6 GE with the JJ5AR4 along with the 6DJ8 is a great combo. If you were to go to a vintage 6L6G coke bottle shape tube you would want to use a 5Y3 rectifier tube. Other 6L6 vintage NOS best suited with a 5U4 rectifier. The 5AR4 is AOK with any of the larger tubes like the KT88, 6550, KT90, KT120 and KT150.

I think it is interesting that while my amp has the 9 pin miniature input tube socket, he is now making then with 8 pin octal socket (e.g., 6SN7). Does anyone know if there is a reason?

I just checked – he has a KT88 amp listed now …

[Rosco65]

Originally Posted by robert_kc

My Inspire amp originally shipped with:

• KT-88 GENALEX OUTPUT TUBES
• 6CG7 NOS INPUT DRIVER
• 5U4 Shuguang RECTIFIER TUBE

Following is one of Dennis’ emails about tube compatibility in my amp:

GT6L6 GE with the JJ5AR4 along with the 6DJ8 is a great combo. If you were to go to a vintage 6L6G coke bottle shape tube you would want to use a 5Y3 rectifier tube. Other 6L6 vintage NOS best suited with a 5U4 rectifier. The 5AR4 is AOK with any of the larger tubes like the KT88, 6550, KT90, KT120 and KT150.

I think it is interesting that while my amp has the 9 pin miniature input tube socket, he is now making then with 8 pin octal socket (e.g., 6SN7). Does anyone know if there is a reason?

I just checked – he has a KT88 amp listed now …

My KT-88HO shipped with a 6DJ8 type. I purchased it January 20, 2016. I think maybe a couple of months later he switch to an octal driver tube. I asked him about this, and his reply was: "The circuit is very similar except for the bias values on the 6SN7 driver on the front end of the KT88 HO. The 6SN7 has a bit more dynamic drive. You may give the 6CG7 a whirl in your amplifier. Similar to a 6SN7 except for the different bias on the 6SN7 driver."

This is interesting, since your amp came supplied with a 6CG7, which is a 6SN7 in a 9-pin bottle. His implication that they would be biased differently puzzles me.

I do know his KT88 amp is now $45 more expensive, and that may reflect a higher parts cost. It also appears that as his amps (and preamps) are becoming more popular he is going a little more upmarket. IIRC, Inspire started as a kitchen table retirement hobby business, originally revamping Magnavox console amps and later producing his own adaptation of the circuit. His metalwork has improved and he now uses higher end passive components. Both Cary Audio and AES exclusively used octal tubes, at least I don't recall seeing 9-pin tubes except as the output buffer in the PH-1 phono stage. It could be that he's switching based on the higher perceived quality of the octal tubes.

That being said, there appear to be a lot more options for affordable tube-rolling with 6SN7's. 6CG7's weren't made by many manufacturers and the best 6DJ8 types have gotten expensive (and frankly was not even designed as an audio tube to begin with). It seems like it is harder to find a good sounding 6DJ8 than a good sounding 6SN7. 6CG7's are electrically identical to 6SN7's but with the elements closer together in the smaller bottle there are potential compromises.

When I saw Dennis had moved to a 6SN7 right after I bought my amp I was disappointed. My options are to use what I have (and am currently happy with), preferably with a 6CG7, send it back to him to get converted to an octal socket, sell it (I have a number of amps), or buy a socket adapter to use a 6SN7. I may do the last anyway just to see how things sound.

[Bombadil]

I spoke to a very experienced tube amp builder and asked about subbing a 6SN7 for a 6DJ8 using an adapter and he told me it would be a terrible substitution. That the two tubes were electrically quite different and it could significantly alter the sound. A circuit designed for one would be ill served with the other. It is worse in the other direction, as the 6SN7 circuit could have much more voltage than a 6DJ8 can handle.

[Rosco65]

Over on another forum, Jim McShane differs:

"Slow down! Relax, deep breaths...
The only difference is pin 9, which in a 6CG7 and a 6DJ8 are both connected to an internal shield in the tube. In a 6FQ7 pin 9 is not connected, so the basing is not an issue at all. No rewiring is needed.

The biggest potential problem relates to plate voltage limits, 130 on the 6DJ8 vs. 330 on a 6CG7/6FQ7 (source - GE Essential Characteristics). Depending on the circuit, the plate may not see a real high voltage. Keep in mind also that the 130 volt spec was the "Design-Center" spec., the most conservative of the three rating sytems used for tube specs. If it were rated using the "Absolute-Maximum" spec., it would be rated considerably higher in all likelihood.

I'd prefer the 6CG7/6FQ7, but YMMV."

[Simonatsea]

Originally Posted by Comzee

After a few weeks of ownership, I'm loving my fire-bottle so much I want to explore the headamp version (Inspire-IHA1). I see on ebay Dennis every so often sells some headamps. I'm in zero rush to get one, because I have plenty of alternatives. Would you guys recommend I wait for Dennis to list a headamp on ebay, or just go for the Moon Audio version?

I would contact him directly, as an Inspire devotee you might actually get a better deal than the general public. I too am interested in the head amp. I currently run my Mrspeaker Alpha dogs directly off the speaker taps on my 6V6 amp.

[Simonatsea]

Has anyone upgraded their amps or preamps from Dennis. I am currious about sending in my LP-2 w/ DACT to be upgraded to the LP-3. Anybody gone for this option? I would be worried that some of the magic might be lost....

[Rosco65]

Originally Posted by Simonatsea

Has anyone upgraded their amps or preamps from Dennis. I am currious about sending in my LP-2 w/ DACT to be upgraded to the LP-3. Anybody gone for this option? I would be worried that some of the magic might be lost....

Is the LP-3 an octal version of the LP-27a, or is the circuit different? I understand that the LP-3 has three 6SN7/6SL7 tubes. However the 6**7 types are double triodes, where the 27/56 tubes in the LP-27a are single triodes. Is Dennis simply running 1/2 of two of the tubes?

[BearCityUSA]

Originally Posted by Simonatsea

Has anyone upgraded their amps or preamps from Dennis. I am currious about sending in my LP-2 w/ DACT to be upgraded to the LP-3. Anybody gone for this option? I would be worried that some of the magic might be lost....

I had Dennis upgrade my older lip-2 with 6dj8s in front and a 6cg7 in back. He said I would see a noticeable improvement. He was right. I do not think you will be disappointed. I have ultimately settled back to the same tube complement I was running prior after shuffling in 6cg7s everywhere and different 6922 variations as well. That is volshod 6h1n eb output tubes, GE 6cg7 driver and Sophia 274b rectifier. A fantastic sounding preamp.