bass amp tube question

[EWBrown]

That would be correct, choose a primary impedance which is close to a third of a single pair P-P impedance. It doesn't have to be exact, just pick the closest available one.

The new K-16 amp from S5 uses the "reflection" trick to match up their existing 9800 ohm:8 ohm (35:1 turns ratio) to 5K which is what PPP quads of 6AQ5 / 6005s work into best. THey spec it as having a 4 ohm outout (though they say anything between 4 and 8 ohms will work). This "reflects" the 4 ohms back to the primary as being 4900 ohms.

The little UTK trannies will be straining at teh cloaca to deliver 16 watts, I'd SWAG that the freq response will lose an octav or two.

I decided to go with Hammond 1615s, a "true" 5K : 4, 8, or 16 ohms. with UL taps which may get used in a future mod / upgrade.

Warning, a little OT content follows:

I might just be putting some UTK OPTs up for pre-bay "adoption" soon, After this project, I'll have eight or ten of them "hanging around"...

They're fairly decent for 8-10 watts. a perfect match for a single pair of 6AQ5s or 6V6s. Oh, yeah, they'll work just fine with any of the 'BM8 or 'GV8 family, 11MS8s, a16A8s, 8B8s, nd even with PP 6EM7s or 6DN7s and their derivitaves.

I did a DCR measurement of the UTK's primary, it's 157 ohms P to P, 83 ohms on one side of B+ , and 74 on the other, this reflects the simple layered "solenoid" winding technique, these definitely don't have any "fancy" winding tricks...

The color code / phasing is the opposite of what Hammond uses, swap blue and brown around, if you need to use NFB.

/ed B in NH

[dhuebert]

Taking this approach might work better with a bigger transformer, a la' the 1650W, since it is rated at 280W

The 1650T is rated at a max DC current of 403 mA. Divided by 6, this gives us 80 mA per bottle. This seems like lots to me, especially since we're not doing HiFi here.

Typically, the BFA power transformer runs at "blazing" during performance and the output transformer runs at "cucumber". Now I know this highly scientifical evaluation dosen't give distortion figures such as "juicy evens" or "80 grit odds" but my tendency would be to keep the 1650T for all future work and go bigger for the power supply.

Don

[dhuebert]

1 mm = ~ 40 thousandths of an inch, that's a fair amount of gap for a vacuum, Jennings says contact spacing in a vacuum relay can be as close as 1000 volts per .001 inch, but there's more going on in a tube than a relay ;-)

For SOME REASON bass players have this ANNOYING HABIT of putting my amps on top of the speakers! I have spoken to them about this SEVERAL TIMES. If this practice continues I may have to TAKE ACTION.

Don

[gerryc]

Taking this approach might work better with a bigger transformer, a la' the 1650W, since it is rated at 280W

The 1650T is rated at a max DC current of 403 mA. Divided by 6, this gives us 80 mA per bottle. This seems like lots to me, especially since we're not doing HiFi here.

Typically, the BFA power transformer runs at "blazing" during performance and the output transformer runs at "cucumber". Now I know this highly scientifical evaluation dosen't give distortion figures such as "juicy evens" or "80 grit odds" but my tendency would be to keep the 1650T for all future work and go bigger for the power supply.

Don

I take it that the OT runs relatively cool. That's good. It just might work. The worst that could happen is it just starts petering out and doesn't deliver the power. However, Kevin O'Connor of London Power has a guitar amp design in his Principles of Power book with a 6550 sextet (not UL though) rated at 200 watts into a 1650T. Also, on their website is a page listing Hammond transformer applications / power transformer matching:

http://www.londonpower.com/hammond/matching.htm

Go to the bottom and look at the section "Extended operation for Musical Instrument Amplifiers". He lists the 1650T as being able to handle 200W but states "Output power is increased by sacrificing low frequency response". So, I don't know, I guess it's worth trying. I've been toying with that idea too since 9.5 lbs of iron is a lot less than 28 lbs in a portable amp (and also half the price)! Since you've been getting good results at 160 watts, pushing to 200 might be doable, if that's the range that you're aiming for.

[jlaney]

I really appreciate this thread targeted at building amps for bass guitar! I'm currently building a PP 6550 70 Watter, before trying a PPP brute.

Regarding screen resistors, does the advice about using values in the 1k range apply equally when using ultralinear taps? I consulted the TUT3 chapter and it is unclear if he is talking about UL.

[gerryc]

I really appreciate this thread targeted at building amps for bass guitar! I'm currently building a PP 6550 70 Watter, before trying a PPP brute.

Regarding screen resistors, does the advice about using values in the 1k range apply equally when using ultralinear taps? I consulted the TUT3 chapter and it is unclear if he is talking about UL.

Looks like there are more of us working on tube bass amps than I thought! Yeah, he doesn't specifically mention ultralinear in TUT3, probably because he's presenting circuits of well known MI amps and how to improve / clone them, none of them using ultralinear configuration. However, in his Principles of Power book he has a number of example ultralinear circuits that I think are targeted more to hi-fi than musical instruments which show a 1K screen resistor to the OT taps. So, I'm going to go out on a limb and infer that it applies to UL as well.

[dhuebert]

Go to the bottom and look at the section "Extended operation for Musical Instrument Amplifiers

HAH! he puts the 278CX together with the 1650T for 160 watts. I did not see this before I built the BFA, coincidence? I can't imagine 6L6 surviving this tho..

Regarding screen resistors, does the advice about using values in the 1k range apply equally when using ultralinear taps

I would think that this goes especially for UL. In pentode mode the screen grid voltage can be set in the power supply, whereas in UL the screen grid actually sees a higher voltage than the plate. In pentode mode the screen grid resistor is more to reduce the high frequency response of the pentode (beam power) reducing the likelyhood of oscillation, whereas in UL we are trying to reduce the power dissipated by the screen grid as well as lower the frequency response. It is my understanding that just about any value is good here but more resistance helps to prolong tube life by lowering screen grid dissipation, within the bounds of resistor power rating and output frequency response.

Output power is increased by sacrificing low frequency response

Its important to remember here that the amp frequency response is rated using a continuous sine wave at full power. This is a situation that will never arise in actual use. Try to imagine a circumstance where you will be playing and the amp will be flat out, a hard slap on the open E? There will be some frequency limiting and distortion when the transformer saturates but this will be fleeting and might actually sound good.

Don

[gerryc]

Output power is increased by sacrificing low frequency response

Its important to remember here that the amp frequency response is rated using a continuous sine wave at full power. This is a situation that will never arise in actual use. Try to imagine a circumstance where you will be playing and the amp will be flat out, a hard slap on the open E? There will be some frequency limiting and distortion when the transformer saturates but this will be fleeting and might actually sound good.

Don

That is true. I wonder how it would fare with a low B-string (I have a 6-stringer as well). I think it's worth some experimentation. I'm leaning towards trying the 1650T with a quad of KT88's with a somewhat higher plate voltage, between 550 and 600 volts to get 200 watts. I like the idea of sticking with four output tubes, and from the datasheets I've seen (not just the old Genalex, but also for current JJ and Shuguang tubes), they're good for 100 watts a pair at around 560 volts. If that works, it'll make quite a nice amp with a reasonable weight. Not quite an SVT, but still pretty potent. By the way, unless one goes for the more "exotic" KT88's, they seem to be priced about the same as 6550's. What brand are you using in your BFA?

[Ty_Bower]

I'm leaning towards trying the 1650T with a quad of KT88's with a somewhat higher plate voltage, between 550 and 600 volts to get 200 watts.

Sorry to drift so far off topic, and I apologize for raising questions in areas where I know so little... but where are you going to put 200 watts of output? Do they make drivers that can tolerate that kind of abuse?

[EWBrown]

Typically, for those power levels, the speaker cab will contain four or eight drivers in a series-parallel arrangement. (8 X 10 would mean eight ten inch speakers, etc) more cones, moves more air :o

HTH

/ed B in NH

Just wondering, what is the standard tuning on a six-string bass?
IIRC, a four stringer is E, A, D, G; would a six stringer be B, E, A, D, G, C (or would the last one be a B?)

[EWBrown]

The 1650T is rated at a max DC current of 403 mA. Divided by 6, this gives us 80 mA per bottle. This seems like lots to me, especially since we're not doing HiFi here.

403 divided by 6 is more like 67.16 mA, it might be good knock 'em back to around 60 mA per, or even 50 mA per, for a little safety margin. Yellow_Light_Colorz_PDT_11

Typically, the BFA power transformer runs at "blazing" during performance and the output transformer runs at "cucumber". Now I know this highly scientifical evaluation dosen't give distortion figures such as "juicy evens" or "80 grit odds" but my tendency would be to keep the 1650T for all future work and go bigger for the power supply.

How about two of the same type of power trannies, each with their own set of rectifiers, then connect the raw DC outputs parallel into the existing CLC filtering. The Hammond PTs seem to run hot anyway, maybe splitting the input power the load would reduce the iron's operating temp from "blazing" to merely "simmering" Yellow_Light_Colorz_PDT_01

Mighe be a better approach than a single "BFT" in the "BFA"... Yellow_Light_Colorz_PDT_02

I always wondered about the practice of placing the amplifier on top of the speaker cabs, it seems like a good way to shorten tube life, and in those amps with upside-down chassis, with all the tubes and iron hanging down, that could be a "recipe for disaster" as the LF vibrations loosen up all of the mounting hardware... :o

/ed B in NH

[dhuebert]

403 divided by 6 is more like 67.16 mA

Sheesh! Lucky I wasn't doing the math on the moon landing!

What brand are you using in your BFA?

I am using a set of matched Sovteks from www.thetubestore.com. I haven't had a failure yet but in the first couple of months they drifted quite alot, I showed up one night and all the plates were glowing red. I rebiased and had to make them colder than I would have liked due to one hottie. Matching was way in the past. I have heard alot of good about the JJs and will use them next.

Don

[gerryc]

I am using a set of matched Sovteks from www.thetubestore.com. I haven't had a failure yet but in the first couple of months they drifted quite alot, I showed up one night and all the plates were glowing red. I rebiased and had to make them colder than I would have liked due to one hottie. Matching was way in the past. I have heard alot of good about the JJs and will use them next.

Don

The way around this is to set up separate bias pots for each tube (or maybe one bias and one balance per pair, same number of pots). I know it's a bit more complex, but it gives you a lot of flexibility without worrying about matched tubes or what to do when they drift away from each other (Kevin O'Connor warns about this by the way). That's what I'm planning to do.

[dhuebert]

The way around this is to set up separate bias pots for each tube

I certainly thought about this, but it's all a learning experience and I wanted to know how well a single bias pot would work. Garnet never worried about bias pots, he set his amps up close enough is good enough and used to tell me to get rid of the pots altogether. His experience was that mismatched tubes sounded better. I know from working on his amps that he would bias them cold so any tube would be within its safe operating range. The crossover notch is huge but the owners love it.

Don

[Gingertube]

Hi guys,
There are a couple of comments / opinions in the thread above which are just plain wrong and I can throw a bit of light elsewhere.

The Hammond 1650T is rated at 120W from 30Hz to 30kHz for HiFi use.

That means it can handle 240W at 60Hz. The Bottom E on your ordinary guitar is 81 Hzand on your bass bottom E is 40.5 Hz - That means you probably won't get 200W from a bass bottom E string pluck BUT you don't want to. Bass sounds tighter if you round off the bottom end a bit and accept a bit of loss when thumping on that bottom E string. My opinion is that the 16050T will be fine.

I've actually comfirmed this - In a 4 x KT88 Ultralinear HiFi Amp at +510 Volt rails, driving a 1650T I did a frequency response run at 50 Watts Output and it was flat down to below 20Hz with distortion due to output transformer core saturation setting in at about 18Hz. By the same working as above that means output transformer core saturation is going to happen at 36Hz at 100W and 72Hz at 200W. So build a 200W Amp and let it roll off by 3dB at around 70Hz. That amp delivered 122W at 1kHz and 135 W in hard overdrive.

Next there is a comment that adding more tubes will not help unless the voltage rail is increased. Thats not right - 3 pairs of tubes will deliver 1/2 again as much current into the transformer as 2 pairs - giving you 1/2 again as much power with the SAME voltage.

Tube choice can also have a moderate influence on power in that different tubes have different saturation voltages (how close they can pull their anodes toward 0V on their cathodes)

KT88, 6550 have saturation voltages up around 90V, with a 510 volt rail that means a 420V peak swing at the anode.

6L6, EL34, KT77 are generally down around 60 volts which allows an extra 30 Volt signal swing on each of the push pull sides - that gives about a 12% power increase just by changing the tubes

Going to the rediculous - Small tubes like 6V6 and EL84 are down around 40V BUT you'd need 12 pairs of these to run a 200W amp (It would and may even sound great BUT you'd need to put wheels on it....).

Now 3 pairs of 6L6GC or KT77 is not going to give you 200W - more like 165 Watts.

4 pairs is what you'd need for 200W - I STRONGLY doubt that you will notice any difference between a 165W Amp (like the old Fender SuperTwin) and a 200W Amp.

If you want a RECOMMENDATION I'd say drive a 1650T with 3 pairs of 6L6GC or try KT77.
I like the voltage rating of the KT77 and I can confirm that they sound stunning in a HI Fi Amp at 420V BUT have never tried them "pushed".
EL34 will give you only around 150W max.

If You really MUST have 200W down to the bottom E string (40.5 Hz) or even lower if you use a 5 string bass then go and see you bank manager for a loan and run 3 or 4 pairs of 6550 into the Hammond 1650W (280W rated at 30Hz) and put aside a bit of the borrowed cash for the chiropractor you'll need to fix you're back after lugging it around.

The reason for the RECOMMENDATION above, apart from the things stated, is that you can pull a proven power amp section design straight from the old Fender Super Twin Schematic - It used 3 pairs of 6L6GC in Ultralinear to give 165W.

I would however put in separate bias controls for each output tube. I will never build another HiFI or Guitar Amp without separate bias controls.

Cheers,
Ian