Tube Experts I need Your Advice

[bassboysam]

I am in the process of possibly having a custom amp designed loosely based on an Ampeg B25. The designer has suggested using 4 x 7591 tubes in the power section. The amp will have a half power switch to run at either 50 or 25 watts. I am not very familiar with this tube and there isn't a lot of information on it. What are its sonic characteristics? It seems to be mostly used in hi-fi amps. Are there any other tubes you would suggest?

The amp will be tube rectified and have a tone stack compromised of the ampeg Baxendall circuit combined with a Fender style rotary graphic EQ to control the mids. Looking to use it mostly for home and studio.

Thanks

[B-string]

Is this to be a guitar, bass or general audio use amp?

[bassboysam]

Sorry, low powered bass amp.

[JimmyM]

I don't know much about them, but I don't know that I'd want a tube that's fairly rare. JJ and EH make them, but there aren't a whole lot of them floating around the NOS world.

[billfitzmaurice]

Originally Posted by bassboysam

The amp will have a half power switch to run at either 50 or 25 watts.

Don't bother, that's a totally unnecessary feature. It has some merit in a guitar amp, but only if you could choose between 5 and 50 watts.

[bassboysam]

So the amp won't distort significantly sooner at 25 watts compared to 50? I wonder if it would be possible to drop it to 5 watts?

[B-string]

7591 were/are a very nice sounding Hi-Fi tube, new production for these though??? Don't know if I ever heard these pushed into live bass guitar duty. KT66 or 6L6 family would be my choice. Tube rectifier for a bass amp when SS diodes perform so much better is not a good plan IMO (guitar amp yes). The 6SL7, 6SN7's should be used as well as 7199's in preamp and tone stack (depending on vintage).

[B-string]

Originally Posted by bassboysam

So the amp won't distort significantly sooner at 25 watts compared to 50? I wonder if it would be possible to drop it to 5 watts?

The difference between 25 and 50 watts would be noticeable but not greatly. 5 watts from a bass amp? Not worth the power to light the filaments.

[bassboysam]

He is suggesting a 6u8a in place of the 7199 and a 5ar4 rectifier.

Can you explain why a tube rec is not recommended?

[JimmyM]

Because B-string 2 doesn't like tubes.



A SS rec will make the amp a little tighter sounding and will eke out a little more clean power. The SVT has a SS rec and it still sounds great, but my B-15 has a tube rec, and I've flirted with trying a SS rec but I never do because the amp sounds great to me as is.

[bassboysam]

I'm looking for that sweaty overdriven sound so that's why I'm thinking tube rec. he said we can compensate a little to keep the low end under control with an over spec'd output transformer.

[bassboysam]

So i guess the question is do you lose any magic by going with 7027/6L6/KT66 tubes instead in the power section?

Any other suggestions for power tubes?

[B-string]

Tube rectifiers are inefficient from the start and suffer voltage sag under high demand. This voltage sag get worse as the tube ages. You can't build a lot of reserve with filter caps as rectifier tubes and electrolytic caps don't play well together. The voltage sag causes loss of low end definition and power (low end gets "flabby"). Loss of high voltage also increases distortion. If you are looking for weak, distorted, hollow bass then tube rectifier is you ticket. It was difficult to design power supplies using tube rectifiers that could hold up well enough for bass guitar use (usually required dual rectifier tubes).

[1958Bassman]

Originally Posted by bassboysam

He is suggesting a 6u8a in place of the 7199 and a 5ar4 rectifier.

Can you explain why a tube rec is not recommended?

Think of power supply voltage as if it's a clothesline. The more demand, the more it sags. A tube will sag, two in parallel will sag a lot less and diodes won't sag much, if at all. You can also do a hybrid rectifier and if the tube fails, the diodes will still be there.

If you use an Electro-Harmonics 5AR4, it doesn't drop the B+ the way the old rectifier tubes did, so you could count on that to keep the voltage more consistent unless you're digging in for a long time.

If you want dirt, the 7591 may be a bit too clean. Look at the tubes used in the amps you really like and use those. A 50W Fender is a nice amp if you want that kind of sound and the 6L6 is very easy to find, old or new.

[B-string]

Originally Posted by JimmyM

Because B-string 2 doesn't like tubes.



A SS rec will make the amp a little tighter sounding and will eke out a little more clean power. The SVT has a SS rec and it still sounds great, but my B-15 has a tube rec, and I've flirted with trying a SS rec but I never do because the amp sounds great to me as is.

LOL you a funny boy

Careful if you try a SS rectifier in the B15 as the B+ can go up as much as 20%. Some plug and play (built into a tube base take that into consideration and drop through a resistor).

[ampegfuzz]

Jimmy might know because I don't, but did the b25 have a SS rec?

[JimmyM]

Originally Posted by B-string

Tube rectifiers are inefficient from the start and suffer voltage sag under high demand. This voltage sag get worse as the tube ages. You can't build a lot of reserve with filter caps as rectifier tubes and electrolytic caps don't play well together. The voltage sag causes loss of low end definition and power (low end gets "flabby"). Loss of high voltage also increases distortion. If you are looking for weak, distorted, hollow bass then tube rectifier is you ticket. It was difficult to design power supplies using tube rectifiers that could hold up well enough for bass guitar use (usually required dual rectifier tubes).

Right, that's why the B-15 sounds so crappy when you crank it.

[1958Bassman]

Originally Posted by bassboysam

So i guess the question is do you lose any magic by going with 7027/6L6/KT66 tubes instead in the power section?

Any other suggestions for power tubes?

6L6 and KT66 are very different animals. Same for EL34/6CA7 vs 6L6.

Maybe a master volume control would be something to consider.

[ampegfuzz]

I still wish I got a later v4 with master instead of my v4b.

[bassboysam]

The amp would operate in 2 modes, master and non-master. Personally I like my pre to be relatively clean and get the dirt from the power section. So turning up the gain to get preamp tube distortion in totally not the point of this endeavor.