That would be 52.5 watts per 6550, right in line with the SVT at 50 watts per 6550.
So why would the Shaw B150 with four KT88 power tubes be rated at only 150 watts?
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That would be 52.5 watts per 6550, right in line with the SVT at 50 watts per 6550.
So why would the Shaw B150 with four KT88 power tubes be rated at only 150 watts?
okcrum
in your chest
I have a 4 holer homebrew amp here that I've run both 6550 and 6CA7 in. It runs about 550V B+ using the Hammond 120W-rated output transformer. I've also found that the output power is tube-limited for the 6CA7s, but OT-limited for the 6550s.
For sure, actual playing (average) power is going to be lower than continous RMS power (but not for static plate dissipation; that's pretty constant). By how much depends on genre, effects, technique...that gives you some design advantages, like the ICAS/CCS ratings for transmitting tubes, but by oral tradition instead. Leo had company; Peter Traynor and Jim Marshall pushed things too. It was like a guitar amp arms race for awhile. Traynor had a 200W design (the Super Custom Special) with 4 6KG6/EL509 TV sweep tubes, because they were cheaper than purpose-built audio tubes and would handle 900V on the plates.
okcrum
in your chest
Those west amps are something, I have a friend with a filmore. Used it for recording recently and I really dug the tone. I'm not usually big on UL as it tends to feel stiffer and less responsive, but really my main experiences were with fender UL designs and I've heard more than one person say don't judge UL by fender.
I didn't see this mentioned. In class AB each tube only conducts 1/2 of the time, so in theory the output power is double of what a single tube can output. In real application that doesn't happen but the output power may be greater than sum of the output of the tubes.
okcrum
in your chest
Actually that's class B, strictly speaking. In AB1 operation, the tubes conduct a little more than 1/2 the time to minimize distortion, but they never conduct grid current (in which case they'd be AB2).
The Classic 400 runs 675v on the plates, so it needs a burly tube!
I think that SVTs are similarly "abusive".
When I retubed my Classic 400, I installed JJ KT-88s, biased them at 70%, and measured a smooth 40v sine wave at 40 Hz into a 4 ohm load -- so a CLEAN 400 watts, at the fundamental frequency of a low E...
Needless to say, my dummy load heated up quickly...and I wanted to plug into something more audible anyhow!
Regarding ultralinear operation, I've been a Talkbass member since 2009, and based on various posts I have read over the years, the general sentiment regarding ultralinear operation seems to be that it doesn't sound as good as nonlinear operation. Any comments on that, anyone?
I find that in general ultra-linear is good in a hi-fi, not so good in an instrument amp. You loose rawness, it is more controlled. That rawness comes from distortion that the ultra-linerar design removes.
Having said that, it comes down to the amp design. Some ultra-linerar musical instrument amps sound good as has been mentioned. So not all ultra-linear amps are the same, the transformer designs differ.
I had a 70's Fender 135W twin reverb that had the UL transformer taps, it sounded great with a telecaster. Loud and bell like tone.
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UL uses feedback from the output transformer primary windings connected to the power tube screens to reduce output distortion, but this also has the impact of reducing harmonic content in the output stage and I believe how the amp responds to load conditions (ie frequency variation of the speaker impedance). Where on the OT primary the UL connection is made plays a role in the degree of THD reduction. There's some interesting details here on design and implementation. The west amps as mentioned, the later 70s silverface fenders (bassman 70/135, 135 watt twin, etc), and sunn amps all used UL output stages.
okcrum
in your chest
The amps are still (mostly) linear in normal operation until they start clipping, then they're going nonlinear. Ultralinear is a special case of distributed loading using OT screen grid taps which gives lower distortion levels comparable to triodes and higher power levels comparable to pentodes/beam power tubes. The actual term "ultralinear" was only used in the patent filed for its use to describe the optimum tap point on the OT primary, typically 43% for KT88 and similar tubes.
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Well, there may be some bassists out there who prefer UL operation for their sound, but one thing I have noticed about myself is, I tend to have a bigger smile on my face when I am experiencing just a little bit of power tube saturation in my sound. So, me thinks I am one of those bassists who, if given a choice, would go with an all-tube amp that is *NOT* ultralinear.
agedhorse
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Ultra-linear topology does nothing at all to prevent power tube saturation.
All this talk about what is and what isn't responsible for a given power output is ignoring the elephant in the room... proper matching of the plate to load.
Many transformers being used provide only mediocre impedance matching, which can be responsible for a 30% loss of output power. Matching is tricky, because different topologies can result in different plate impedances.
All this talk about what is and what isn't responsible for a given power output is ignoring the elephant in the room... proper matching of the plate to load.
Many transformers being used provide only mediocre impedance matching, which can be responsible for a 30% loss of output power. Matching is tricky, because different topologies can result in different plate impedances.
okcrum
in your chest
So far, we've assumed perfect matches and lossless OTs, actually. Did someone say ultralinear operation prevented clipping? I don't remember that. Heck, lots of us have heard Mel Shachter live. I'll admit I didn't mention that it lowers the plate impedance. I did refrain from saying it hybridized elements of voltage amps and transconductance amps, because that might be a little much for page 2 of the thread.
Agreed, OTs are more than a thread (and network) unto themselves.
Conversely, the load impedance is transformed to a primary impedance by the OT that hopefully comes somewhere near the combined plate impedances of the output tubes. Different cabs can yield different plate impedances at different frequencies. 30% power loss implies something like a 3.5:1 impedance mismatch, though - that's pretty severe - and something is going to get hot!
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agedhorse
Commercial User
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This is the post I was referencing. Ultralinear doesn't have anything to do with preventing power tube saturation.
Beans is the B-15S the same, could I run a pair of 6550's in it instead of the 7027A's get a little more power and longer tube life, are they plug and play?
"Reducing harmonic content." I think this may be why there seems to be a general sentiment among some TB'ers that nonlinear amps have a more pleasing bass amp sound than UL amps (depending, of course, on the player's tonal preferences).
They don't mention this in the brochure and I've never tried running 6550's in that amp. One issue is the taller tube fitting under the cage. Another possible issue is the higher heater current that the 6550's draw. The power transformer has to be capable of supplying more current, 0.9A vs 1.6A @ 6.3VAC. So the transformer has to be capable of supplying and additional 2.8A. I don't have the specs of the power transformer heater winding. A reissue transformer such as the one that fliptops sells may be able to handle the additional current draw, and on the plus side, they can confirm this for you. I wouldn't recommend doing this with the vintage potted transformer. They get hot enough with the 7027A tubes.
https://www.talkbass.com/attachments/b15s-brochure-pdf.981203/
I know that good 7027A tubes are harder to find. There is an option, the tung-sol 5781A ( www.thetubestore.com - Tung-Sol 7581A Audio Tubes ). I've been very happy with the performance of this tube in my vintage V4B. Price is good, the tubes are well matched. It has the same heater current draw as the 7027A.
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