HDRX-20 Could use some BIAS advice and thoughts on this.

gamjamm

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Recently purchased the HDRX-20. It came with matched Tung-Sol 5881's. They were factory set to 27.5ma bias. The tubes were well matched to 1/2 of one ma. apart. They sounded better to me at 27.5ma although I did try the recommended 29ma which distorted better. I would like to try out a matched pair of Sovtek 5881/6L6WGC tubes. They have the wafer {disc} bases. Do I stick with the recommended 29ma bias? These are Russian 6p3s-e equivalent tubes with a 30watt rating vs the Tung-Sol 23watt. Plus, the Sovtek's are mil-spec., the Tung-Sols are not. I don't expect or need any more watts or stress to the tranny but I'd like a bit more clean headroom and reliability they seem to provide. I do have some spring retainers because the bear traps will rest on the glass {due to the short base} and I'm particularly not a fan of them. Any thoughts/experience will help Thanks, Chuck.
 

Rider1260

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All the PRS amps I have for worked on seem to be right at or near 30ma bias ( MT15 , H ) I have run EL34s in the MT15 and have EL34s in my H
With an amp that new I would 110% contact PRS they have been very open to recommending Bias and tube types and values
 

gamjamm

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All the PRS amps I have for worked on seem to be right at or near 30ma bias ( MT15 , H ) I have run EL34s in the MT15 and have EL34s in my H
With an amp that new I would 110% contact PRS they have been very open to recommending Bias and tube types and values
Thank you. Did the EL-34's add any headroom on the MT-15 compared to the stock tubes?
 

gamjamm

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Thank you. Did the EL-34's add any headroom on the MT-15 compared to the stock tubes?
BTW: I contacted PRS via their support portal. They will e-mail me. Asked for any bias input recommendation/tube compatibility or power transformer voltage issues. The power tranny looks just as beefy as my 50watt tube midget was but that doesn't mean much. The Sovtek's can handle more plate voltage than the Tung-Sols so that should not be any issue.
 

gamjamm

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Happy to report PRS replied. They recommended a 25-30ma bias for the tubes in question. I've personally found that a 27.5ma bias suits my needs. The same as the bias that was set at the factory for the original [very well matched] Tung-Sol tube set.
 

gamjamm

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All the PRS amps I have for worked on seem to be right at or near 30ma bias ( MT15 , H ) I have run EL34s in the MT15 and have EL34s in my H
With an amp that new I would 110% contact PRS they have been very open to recommending Bias and tube types and values
Thanks again. PRS recommended 25-30ma. I've found the 27.5ma setting worked for me the best just as the it was set for the original [and very well matched] Tung-Sols. The Sovteks do seem a bit stiffer as I expected. Your input was spot on.
 

Maggot Brain

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Heads up for others, If you look below the bias points it says the recommended bias right on the amp... Could be easy to miss.

Enjoy ✌
 

gamjamm

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Great point. That said, 5881's from different manufacturers have different recommended bias specs which are also dependent on an amps design {such as varying tube plate voltages, power transformer limits, etc.} The problem is the manual calls for 29ma yet it was set to 27.5ma from the factory. That was the average between both of them. I have to say again that the original tubes were astonishingly close, being only.5 of 1ma apart. My replacements were1.5ma apart which is still well in spec of PRS recommended 5ma max. So once they drift apart to over that you should replace them due to the fact that the amps design only allows the pair to be biased, not the individual tube. PRS recommends 25 to 30ma. Setting it at an averaged 30ma will cause the amp to overdrive the power tubes a little earlier. It's a matter of taste and what you want to achieve. I wanted what some refer to as a "hard" vs "soft tube rating that also had a bias that allowed higher clean headroom. Accordingly, I tried the lower end of the scale but found 25ma was a little too cold. The amp is one of the better ones out there at that price point. The construction and components are better than either the Marshall DSL-20 and sadly even the SV-20 {way over priced} in my opinion. Open them up side by side and I think most would agree.
 

screaming_head64

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I just checked my bias on the HDRX 20 and from factory it was biased with an average of just under 26. Do they always ship them with colder bias? anyway, time to experiment :) love the amp even at 26 bias
 

screaming_head64

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Dec 2, 2022
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Recently purchased the HDRX-20. It came with matched Tung-Sol 5881's. They were factory set to 27.5ma bias. The tubes were well matched to 1/2 of one ma. apart. They sounded better to me at 27.5ma although I did try the recommended 29ma which distorted better. I would like to try out a matched pair of Sovtek 5881/6L6WGC tubes. They have the wafer {disc} bases. Do I stick with the recommended 29ma bias? These are Russian 6p3s-e equivalent tubes with a 30watt rating vs the Tung-Sol 23watt. Plus, the Sovtek's are mil-spec., the Tung-Sols are not. I don't expect or need any more watts or stress to the tranny but I'd like a bit more clean headroom and reliability they seem to provide. I do have some spring retainers because the bear traps will rest on the glass {due to the short base} and I'm particularly not a fan of them. Any thoughts/experience will help Thanks, Chuck.
BTW, you do mean mV don't you? that is how it is measured in mV, not mA.....

I just raised mine to around 28 mV average and it sounds much better.

EDIT: Found the sweet spot at 29.5 ....Wow, what a difference from how it was set at factory.
 
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