Looking to buy a quad of used 6550's for my Allen model 75 mono block power amplifiers. Had recently purchased a new quad of Tung Sol KT 88's for them and I absolutely don't like the sound. So, I thought I would try some used 6550 WE Sovtek tubes and give them a try. I had always liked the sound of this brand in different power amps. So, looking at two different used quads that were measured on a Maximatcher with the following measurements. One quad has specs of 31-35ma/ 4.02-4.28 mMhos and one quad has 43-46ma/4.57-4.97mMhos. Which quad would be better?
Is it possible to be sure? It looks like they were measured nowhere near their normal operating point since normal operating data is 85mA plate current at 400V plate voltage. If the quads are the same manufacturer then I suppose the second set have better emissions, but it is not an exact science.
Maybe measuring them in the amp is better, if that is straightforward for you. Is there a convenient cathode or anode resistor so you can measure voltage drop, and hence current?
Maybe measuring them in the amp is better, if that is straightforward for you. Is there a convenient cathode or anode resistor so you can measure voltage drop, and hence current?
How many hours did you put on the KT88s before you listened? I like to get at least 25 hours before I even start to judge and 50 before I reach any conclusions. Not just sitting idling but playing music, into a dummy load if necessary.
Some people have been known to swap in new caps, fire them up and write them off after 5 minutes. I put a pair of Lundahl OPTs in an amp once and they sounded like crap right away but wonderful at the 50 hour mark. I was flabbergasted by the change.
S.
Some people have been known to swap in new caps, fire them up and write them off after 5 minutes. I put a pair of Lundahl OPTs in an amp once and they sounded like crap right away but wonderful at the 50 hour mark. I was flabbergasted by the change.
S.
The Allens probably have an hour hooked up to the dummy load to monitor them and get them up to a good working temp before setting the final bias and adjusting the balance control. This is probably my 6th pair of 75's. I've kinda played with different brands of caps until I found something that I liked. Will probably keep this pair and avoid inviting others to listen to them as I can never turn down a decent offer to sell and this is my last pair. In all fairness I probably haven't given the KT88's a chance but from the get go they just lack the sweetness of a good 6550. The KT88's have a lot of bass but lack in the mid range and high frequency detail. I will give them another chance and let them play music instead of a sine wave into the dummy load for a day or so and see if it makes a difference. I don't remember the output transformers impedance but I believe its more suited to the 6550's instead of the KT88's. They are in my opinion after capped an amplifier that will hold its own against higher priced units. This is the first time I have used KT88's in them as I have always relied on Tung Sol 6550's in the past.
I went ahead and purchased the second quad of Sovteks and they are on their way. Probably get them in about 6-12 days or so. Will take a look at the bias for sure. Hopefully these will have enough left in them to bias up to specs.
Me too, tubes take hundreds if not thousand hours to show any appreciable change.
Transformers may take tens of thousands if at all.
I would take 25-50 hour changes with an grain of salt.
I mean component changes; users run on a different time scale.
My experience is that the greatest changes happen in the first 50 hours. Granted, there is still more changing the longer things run but usually by 100 or so it's possible to ascertain the direction things are headed. As usual YMMV, a lot.
I usually burn gear in by continuously streaming music, changing it up between various types.
S.
I usually burn gear in by continuously streaming music, changing it up between various types.
S.
Why not burn in using 15Hz? Then you don't have to hear it... You get to watch the woofers wiggle if you want though 😀
The Sovtek 6550's arrived and were installed and bias set on the Allens. What I difference in sound the 6550's made. The 6550's in my opinion seem to sound better thru out. The midrange detail and high frequency detail along with the low frequency response in my opinion seems better. The 6550 is what is supposed to be used in the amp but information on the amps say the KT88's can also be used. My opinion maybe the output transformer is better suited to the 6550 than the KT88's. I was mistaken about the brand of KT88's as the boxes say Mullard. Probably going to be my first and last quad of them.