KT88s and 6550s

Amps & Cabs Deliverance
1

I’ve got a pair of gold lion KT88s in my deliverance 60. they’ve been in the amp for 3 or 4 years now and have a large amount of loud play hours on that. I was doing to preventative maintenance before we go out on the road and figured I’d check bias etc. of course the current tubes biased up fine (135 mA for both tubes with the bias trim in approx middle position) and sound great. I always bring backups tubes with me on the road so figured I’d pop those in and make sure they’re okay too.

  • My backup EHX KT88’s: these have less than 1 hour of play time. I probably bought them 3 or 4 years ago and tested them out then. I don’t remember having any issues. When I put these in, even with the bias trim on MAX (coldest bias) it was still coming in at over 140 mA quiescent current. whereas with the “good” tubes with the bias on MAX it was like 80 mA. so for some reason these tubes are pulling way more bias current than expected. I’m not sure why, and I’m not comfortable running the tubes with that much bias current. like I said, virtually no play time on this pair and the last time they were in the amp they were fine. took some recordings versus the “good” tubes. the sound is still good. with these it was maybe a slight touch quieter.

  • I have another set of gold lion kt88’s with a good amount of playtime on them. I don’t remember taking them out due to any kind of issue. I popped these in after the EHX’s and it was a similar but less extreme issue. with the bias trim on MAX, coming in at 137 mA bias current. leaving me no room for adjustment. again, these tubes were fine the last time I used them. recording wise these didn’t sound too different than the “good” tubes

So I’m left with a few questions. why, on not one but two pairs of supposedly working KT88s, are they suffering from the same issue (pulling way too much bias current) after being in storage?

Second, it’s becoming clear that what I thought was a cache of spares in fact is not tenable, and I need some more spares. tube prices are ridiculous these days. new gold lion kt88s are like twice as much as the last time I bought them. What’s the scoop at fryette on good, reliable KT88s?

Third, I noticed that newer deliverances are shipping with 6550s. This thread gives some advice on 6550 brands: What 6550s do you recommend?
Will there be a noticeable difference between the sound of 6550s and KT88s in my amp? I’m pretty attached to the way it sounds right now.

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That’s been the problem with EH for quite a while now. They sell what they get and what they get is all over the place. And not just by a few mA.

In the past, we specified an Ip limit on tubes we purchased from them - usually no less than 30mA and no more than 50mA - and they complied.

Since 2020 they’ve revised their policy and no longer accept a limit on Ip. As a result, we were seeing everything from 15mA to 75mA. No adjustable bias configuration can easily compensate for a range like that. Now we only buy very limited types from them.

The EH KT88 production was interrupted during this time so we switched to 6550. There hasn’t been a drastic difference in performance, so even though the EH (or any other Russian brand) KT88 is back, we’re not buying them until they reinstate the customer Ip spec policy. JJ KT88s never sounded that good, so we didn’t switch to those.

We currently use Psvane, JJ and Sovtek 6550s in that order of preference.

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