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BillEpstein 13th May 2009 09:25 AM

Tube choices for Simple SE?
I have a pair of used 6550 in mine which now has about 10 hours on and sounds great, except, one tube is at least 3dB louder than the other. So I'm wondering; do I get another pair of 6550 or KT-66 or 88 or...? My speakers are 97dB sensitive so watts don't matter.

Then what to do about the 5W 560 ohm resistor if I change to another tube? I have the Transcendar OPTS, 5000 ohm, with 320Vdc B+ at the plate and 350 at the OPT.

What are Vp, Vk, Rk, Ik on the "tubes and transformers" page? They don't Google.

nic6paul 13th May 2009 11:29 AM

Just curious what power transformer you are using. Totally irrelevant to your question sorry ;)

BillEpstein 13th May 2009 01:45 PM

It's the Allied.

rknize 13th May 2009 04:13 PM

The answer to that question is greatly a matter of opinion. You can read some banter about it on the 3rd page of this thread, which is my Simple SE build:

While the differences between tube types is fairly subtle on my Dynaco PP amp, they are much more obvious on this amp, especially when it is in triode mode with no CFB. What works best for you will depend on your speakers and the type of music you listen to. I also found that the coupling capacitors make a noticeable difference as well. I started with Russian PIOs but now have Auricaps in there.

I think most will agree that a pair of EL34s resolve the most detail with this amp. I have tried a pair of SED EL34s and a pair of cheap Chinese EL34s (Valve Art). They both sound very similar, but I want to say that the SED may have better bass on my older speakers. On my newer speakers, which are 98dB, I can't really tell the difference.

I think that 6550s and KT88s will all sound similar. This is especially true if you get one of the Russian Reflektor tubes (EH, Sovtek, "Tung-Sol", etc), as I think they have all the same innards. I've also tried the EH KT90s, which are supposedly derived from some Russian sweep tubes, and thought they sounded really good. I've had a pair of SED KT88s in there now and I think they are a bit more well-rounded than the Reflector stuff. They have been in and out but I seem to end up going back to them.

I started with JJ 6L6GCs, as these should be a compromise between the detail of the EL34 and the bass drive capability of the bigger tubes. In my opinion they seem to give up too much detail for very little gain in the bass dept. I imagine KT66 would be similar.

Anyway, those are my feelings. Take them with a grain of salt.

Jeb-D. 13th May 2009 04:54 PM


I started with Russian PIOs but now have Auricaps in there.
I tried the Russian PIO's (K40Y-9) in my 6c33c SE. I found the upper mid range and highs to be overly exaggerated. What was your take on them?

I didn't let them burn in very long though. They would probably be a good match for an amp that otherwise sounds rolled off.

rknize 13th May 2009 05:09 PM

There was a noticeable increase in detail with switching to the film caps. My speakers are not very strong in the midrange dept, so I didn't notice any difference there. Since they have horns, the loss of detail was actually a benefit for some types of music. I find Auricaps to be a very neutral cap. They are not as expensive as many others (though they are not as cheap as they used to be) but always seem to do well.

I will admit that I didn't give the PIOs a fair shake. It took a long time, like 50-100 hours, for the Dynaco to sound like it does now. It has those same caps in it. I was too impatient and a little frustrated, so I just swapped them. I always intended to put Auricaps in there anyway, I just didn't have the right value on hand when I was breadbaording the amp.

Ty_Bower 13th May 2009 05:20 PM


Originally posted by BillEpstein
It's the Allied.
Which Allied? The 6K7VG or the 6K56VG?

Judging by your measured B+ voltage, I'd wager it's the 6K56VG. With the lower B+, it might be well suited for some 6L6G or maybe even some 6V6 tubes. If you stick with the big tubes, you're probably running them way underbiased. I'm guessing your cathode voltage is somewhere around 30V, and per tube current draw is probably about 50 mA. That puts your overall dissipation at less than 15 watts. Any decent 6550 should be able to handle much more than that.

rknize 13th May 2009 07:01 PM

Oh yeah...the bias resistor. 560 is a high value to use for a 320V B+, even for an EL34. I use that same resistance on mine for the EL34, but my B+ is 460V. Something more like 390 or 360 ohms will put some current through those things and get them sounding good. That is even more the case for your 6550s. They will do much better with more current and that may be why one sounds so much weaker than the other. Try 270 or 240 ohms to kick the current up.

Ty_Bower 13th May 2009 07:06 PM


Originally posted by rknize
They will do much better with more current... Try 270 or 240 ohms to kick the current up.
The Allied 6K56VG is only rated for 120 mA DC on the high voltage secondary. Remember the 12AT7 is taking about 10 mA per side. It is rated for 50 Hz operation, so you could probably stretch that 120 mA budget a little bit if you live on 60 Hz power, but I wouldn't go too far.

boywonder 13th May 2009 07:11 PM

Re: Tube choices for Simple SE?

Originally posted by BillEpstein

What are Vp, Vk, Rk, Ik on the "tubes and transformers" page? They don't Google.

Vp=plate voltage (determined by B+ voltage and current)
Vk=cathode voltage (determined by cathode R Value and cathode current)
Rk=cathode resistor value
Ik=cathode current (same as anode current for triode mode)

Risking a sweeping generalization, EL34's have great midrange/vocals, and 6550/KT88 give up a little mids for better bass and more punch/attack.

As already mentioned, you may need more PS power to drive 6550's/KT88's than EL34's.

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