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The Un-Official Sadowsky SA200 Thread...
- Thread starter jokerjkny
- Start date
Never saw a preamp need matched pairs (I suppose for a hifi stereo preamp you'd want identical output from each channel).
In most MI preamps each tube (often even sections within the tube) serves a different function within the circuitry & using matched pairs is not required.
In most MI preamps each tube (often even sections within the tube) serves a different function within the circuitry & using matched pairs is not required.
My Reeves uses 1/2 for channel 1 and 1/2 for channel 2 (V1) dual triode.
Awesome!
When you get a chance, can you clarify a few things.
1. Is the bias circuit scaled for KT88s, or will a resistor need to be replaced in the bias supply so more voltage is available? In other words, can you drop in a set of KT88s and bias the amp to spec without a mod?
2. Is the bias spec the same for all tube types? Bias to 25-30mV across the cathode resistor.
A bit of background to explain my questions:
Vintage Marshall 100W heads can be setup for either EL34 or 6550s. When the amps are switched between tube types, the bias circuit must be rescaled and a few other components are typically changed as well. For bias purposes, EL34s are treated as 25W tubes and many techs also treat 6550s as 25W tubes in this application. In other words the quiescent current in the output tubes is the same regardless of tube type. Some techs recommend treating the 6550 as a 30W tube in this application. This means 6550s are biased to pull a bit more current.
Of course tube data sheets for the 6550 show it to be either a 35W or 42W tube, but it is said to be unwise to bias a Marshall to this rating unless you want to melt down the transformers.
I've had my SA200 worked on over the summer by Blackie Pagano, at the suggestion of Roger Sadowsky, and he did indeed install KT88's in my amp.
And yes, the amplifier sounds absolutely sublime to my ears.
Is it fuller sounding in the deep bass region?I've had my SA200 worked on over the summer by Blackie Pagano, at the suggestion of Roger Sadowsky, and he did indeed install KT88's in my amp.
And yes, the amplifier sounds absolutely sublime to my ears.
- Apr 3, 2003
- 35,518
- 19,954
- Disclosures
- Editor-in-Chief, Bass Gear Magazine
I've had my SA200 worked on over the summer by Blackie Pagano, at the suggestion of Roger Sadowsky, and he did indeed install KT88's in my amp.
And yes, the amplifier sounds absolutely sublime to my ears.
Did you notice any increase in volume?
It perhaps is a bit louder, and cleaner (if that's possible) with 1 or 2 SA410 cabs.
Either way, it's a great sound, and IMO the gold standard of "tube clean/pristine"
Either way, it's a great sound, and IMO the gold standard of "tube clean/pristine"
It does seem "heavier" in the deep bass region. Perhaps this is contributing to the slight perception in increased volume.
I'm not sure I like the kt88's any better, but they are certainly different to what was originally intended in the amp.
I was a bit surprised Blackie Pagano (amp guru) put the kt88's in there, as they are not original spec for the SA200, but since he is who is, I decided to work with them for a while, and I'm happy I did.
According to Blackie, he felt the original EL34's did not have the necessary voltage headroom for the amp and that is why they were failing too often.
Here is some additional info from Tube Amp Guru, Blackie Pagano! Thanks Blackie.
Please be aware that the SA-200 is not self biasing. If you are going to switch from the EL34s, please bring it to a good amp technician to install the KT88 power tubes.
Roger
The bias circuit has enough range to adjust for kt88. I would set them around 30 milliamps. The range of currents somewhat depends on the actual set of tubes that are fitted. I recommend buying tubes that are "middle numbers" on the matching scale which should put you in a good range for setting bias in the stock circuit. My recommendation for kt88 was based on the elevated screen voltage which exceeds design maximum for el34's. This is fairly common , but Kt88's are rated to 600v on the screens which makes them happier in circuit and less prone to failure.
By the way, I noted that the original question switches from kt88 to 6550. I definitely do not recommend 6550 in circuit as their screen voltages are rated much lower.
Please be aware that the SA-200 is not self biasing. If you are going to switch from the EL34s, please bring it to a good amp technician to install the KT88 power tubes.
Roger
The bias circuit has enough range to adjust for kt88. I would set them around 30 milliamps. The range of currents somewhat depends on the actual set of tubes that are fitted. I recommend buying tubes that are "middle numbers" on the matching scale which should put you in a good range for setting bias in the stock circuit. My recommendation for kt88 was based on the elevated screen voltage which exceeds design maximum for el34's. This is fairly common , but Kt88's are rated to 600v on the screens which makes them happier in circuit and less prone to failure.
By the way, I noted that the original question switches from kt88 to 6550. I definitely do not recommend 6550 in circuit as their screen voltages are rated much lower.
- Apr 3, 2003
- 35,518
- 19,954
- Disclosures
- Editor-in-Chief, Bass Gear Magazine
Awesome!
I had the same concern with the high screen voltage regarding running 6550s in the amp. Since I have two of these beauties, I had planned to eventually ask about converting one to KT88s. I'll update my notes, so when the time comes I have a good reference.
Thank you sir for the prompt and helpful response. I think not it's not necessary even to change the fuse if going from 100V to 115V - right?
Sounds simple enough for a moderately handy bass player. Or would there be a need to take to an amp shop to rebias?
Sounds simple enough for a moderately handy bass player. Or would there be a need to take to an amp shop to rebias?
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