Hybrid amp for bass & acoustic

[tubenit]

I have a friend, Paul, who is a remarkable musician (vocalist, acoustic, electric, bass, percussion, keyboard, violinist).  He is not a purist electric guitar player meaning thinking primarily about smooth guitar driven overdrive amps.

He has a Mesa Subway bass solid state PreAmp that he wants to use into a tube poweramp.

http://www.mesaboogie.com/amplifiers/bass/preamplifier-series/subway-bass-di-preamp/index.html
http://www.mesaboogie.com/media/User%20Manuals/070543-Subway_DI-Preamp_download_161219.pdf

We also had some discussion of him using his Larrivee acoustic guitar which has a built in preamp into the poweramp.

I got to thinking about this and I am advocating adding one tube and a mosfet cathode follower so he could choose to play his Les Paul copy thru a tube preamp also. 

The idea would be to plug his bass or acoustic into a passive FX return jack into the LTPI and power amp.  OR ......... plug his electric into the input into the preamp.

He will primarily be playing this at home (with young children around) or in a small venue.  I'm thinking 50-60 watts tube power for bass at home or in a small venue would be sufficient with plenty of headroom?

Given the early bass tube amps were 25 watts to 60 watts (with 5F6-A tweed bassman being about 45 watts),  I am thinking that using two pair of cathode biased 6L6's would give him maybe 50-60 watts ?    And that he could simply pull two 6L6 tubes to reduce power (down to 25-30 watts) to play electric or acoustic if he wanted or needed to.  I like the cathode biased idea vs. him having to pull tubes and re-bias a fixed biased amp.

With respect, Tubenit

[tubenit]

My main questions are:

Q1:  Do you see any problem plugging the Mesa Subway solid state directly into the FX return using it as a "power amp input"?

Q2:  I think Paul has two 8 ohm speaker cabs, but I'm not sure?  Could he simply run the amp into a cab with one 8ohm speaker?  I realize doing
       this may reduce power or shift the frequency response somewhat having a slight impedence mismatch? 

Q3:  Would there be an issue of pulling two 6L6's (reducing down to one pair) and running the 8 ohm speaker cab?

Any other critiques or thoughts you guys might have on how to have this idea of a moderate wattage bass tube amp also used for acoustic be successful for him?

I am thinking of this amp having two inputs:  input to preamp  & input to poweramp.

With respect, Tubenit

[tubenit]

So, here's the concept.  If I am understanding him correctly, I think he has an LR Baggs acoustic preamp to DI like this one:

https://www.sweetwater.com/store/detail/ParaDI

He'd also plug that into the FX return going into the LTPI.

With respect, Tubenit

[tubenit]

I'm not really familiar with bass tube amps.  Is what I am proposing in this thread seem realistic?  Do you think 60 watts for a bass tube amp is realistic for playing at home or small venues?

With respect, Tubenit

[davidwpack]

I play guitar a lot through an Ampeg. Past 6 it starts vibrating the windows and walls and is hard to be in the same room without my ears bleeding. It's around 50 watts.

[silverfox]

I've really started appreciating the power conditioning equipment I see offered. The concept anyway since an average band running 15" to 18" is going to suck an awful lot of surge power from the line. That surge is what could make a 60 watt bass amp under perform or get flabby. But in a home environment, I'd guess plenty loud at the lower frequencies. Plenty loud anyway for the home even if it were just guitar.

A significant difference in bass amps would be the coupling caps for the phase inverter. I'd look at some Sun, Marshall or Acoustic amp schematics. Add an EQ circuit? In that regard, the acoustic guitar is going to be in a different EQ range then a bass. Now I'm thinking the circuit should perhaps resemble a mixer or put a small mixer in front of the Return. I've done that before and it sounds good when properly EQ. As an added feature, if he plays other instruments they could plug in too.

silverfox.

[tubenit]

the acoustic guitar is going to be in a different EQ range then a bass

I am thinking his Mesa Subway preamp  for bass and his LR Baggs preamp for acoustic will each have their own suitable EQ in their respective preamps.  IF more tweaking is needed, then an EQ pedal between the preamp and LTPI could be used.

Sounds like 60 watts should be a reasonable watt level for home and perhaps small venue?    I am understanding the power conditioning comment to be about the volts from the wall outlet or extension cord.

Thanks for the comments!  With respect, Tubenit

[John]

Sounds like 60 watts should be a reasonable watt level for home and perhaps small venue?

The little 25/30 watter I built for my buddy gets plenty loud, so I would think yours with twice the tubes should be fine in small venues. Should still sound decent for practice at home too, IMO.

[tubenit]

The little 25/30 watter I built for my buddy gets plenty loud

What kind of bass amp did you build?   OR ............ do you have a schematic that you could share?

This one here seems reasonably loud and bass driven & it's 25 watts :

! No longer available

With respect, Tubenit

[SILVERGUN]

The concept sounds like a great idea.
I think most guys would say that you need more wattage for bass if you're trying to be heard over a drummer (which drummer? - how hard does he play? - other instruments on stage?)...and since you'll have 4 - 6L6s working anyway, you might at well suck the 100 watt capability out of them, which will give you greater headroom and less distortion in the output section.
Higher B+ and fixed bias will help. At least maybe as a switchable option.
 
You can also just switch out the cathode connections (X) to kill 2 tubes, so he doesn't have to physically pull them.
Same scenario is possible with fixed bias because when you shut the 2 tubes off you can still have an appropriate bias voltage on the remaining 2 tubes.
 
If you are not committed to that specific PT, I would upgrade to beefier specs there.
Check out the scratch and dent Twin Reverb iron Doug has in the For Sale section. That will give you the higher voltage and current capability.
 
Obviously, just my thoughts and perception based on what I've read.
Bass likes more clean power.
Some of this will depend on the signal level that comes out of his bass box, how hard he wants to drive it, or how clean he will want it to stay.
 
My comment about the PT is based on the idea that you will not wind up with the 420VDC B+ once those 4- 6L6s start sucking all of the current capability out of that 250mA PT
 
I would also be inclined to experiment with the PI values to take full advantage of the 12AT7, by lowering the plate loads to 47K and 51K and maybe utilizing a balance pot
 

[92Volts]

I've been reading about bass amps because I'm planning a build for a friend... While this article is not real informative for "sane" projects I had fun reading about the Fender 400PS: https://reverb.com/news/the-most-powerful-tube-amp-of-all-time-behind-the-fender-400-ps

One thing I didn't expect/realize which I saw in that article was "...a hi–fi style power amp that can output full power at 40 Hz. The SVT preamp, on the other hand, had a bass roll–off starting at 90 Hz..."

The lowest notes on a bass are well below 90Hz. There is harmonic content above each string's fundamental frequency, so you can (apparently) get away with this.

Rolling off the low frequency (or at least accepting inherent rolloff) lets you put a LOT more power through the same OPT, and it should be perceived as louder for the same power.

[John]

The little 25/30 watter I built for my buddy gets plenty loud

What kind of bass amp did you build?   OR ............ do you have a schematic that you could share?

This one here seems reasonably loud and bass driven & it's 25 watts :

! No longer available

With respect, Tubenit

Jeff, it's basically an Ampeg B-15n clone. I say "basically" because while I used that schematic almost exactly, I only had the 1 channel instead of 2. I used transformers from a hifi mono amp: HH Scott 99 as close as I can recall. The PT was a little undersized when it came to current, but the OT was quite large. I think it weighed about as much as the PT did.


I wound up with 405 v on the plates of the 6L6's, (under load) and got about a 40v drop when I'd hammer a power chord or hit a heavy bass note. Which is why I thought it didn't quite keep up on the current side. I've got it biased at right around 48 mA per tube IIRC.


Attached is my as built schematic sch file.

[sluckey]

The very early Ampeg B15 was cathode biased. Here's a schematic...

     http://el34world.com/charts/Schematics/files/ampeg/Ampeg_b15n_portaflex.pdf

The later fixed bias B15 schematics are also in Hoffman's schematic library.

The B15 is a good choice for a small bass amp. Very popular in small bands and studios back in the '60s.

[PRR]

"Bass Amp" is also the LOUDspeaker. Big air-wave at 41Hz needs a BIG air-slapper to be efficient. If his roadies will carry more than a van's worth of cabinets, not so many Watts are needed. If he's trying to compete with a hot drummer and a Twin, with just one 15 in a small box, it's gonna take more Watts than is reasonable for a tube amp (F400 and SVT were barely reasonable then and impractical today).

What I am seeing with your base plan is that the longtail+6L6 section needs 2V-3V of drive, and that 9V powered preamp can barely make 2V-3V. You may not be able to cream the 6L6es. Which may not be your goal, but it is nice to have a little over-the-top on tap.

[tubenit]

What I am seeing with your base plan is that the longtail+6L6 section needs 2V-3V of drive, and that 9V powered preamp can barely make 2V-3V. You may not be able to cream the 6L6es. Which may not be your goal, but it is nice to have a little over-the-top on tap.

In light of that,  would it be better to have a 12A_7 triode gain stage prior to the LTPI and have the preamp go into that triode gain stage?

You can also just switch out the cathode connections (X) to kill 2 tubes, so he doesn't have to physically pull them.
Same scenario is possible with fixed bias because when you shut the 2 tubes off you can still have an appropriate bias voltage on the remaining 2 tubes.

You're saying IF I shut off the cathode connections on the outer pair of 6L6's, then I can still use fixed biased and NOT have to rebias?  I thought it would have to be rebiased once that pair was shut off?   

with respect, Tubenit

[SILVERGUN]

You can also just switch out the cathode connections (X) to kill 2 tubes, so he doesn't have to physically pull them.
Same scenario is possible with fixed bias because when you shut the 2 tubes off you can still have an appropriate bias voltage on the remaining 2 tubes.

You're saying IF I shut off the cathode connections on the outer pair of 6L6's, then I can still use fixed biased and NOT have to rebias?  I thought it would have to be rebiased once that pair was shut off?   

with respect, Tubenit

In fixed bias operation, when you open the 2 cathode connections the plate voltage on the remaining 2 tubes will rise slightly, and bias current will increase slightly, so if you start on the cold side (let's say 60% diss with all 4 tubes going) it shouldn't rise enough to be an issue.
Using round numbers as an example:
If all 4 tubes are seeing 400V B+ and -40VDC bias voltage and they are running at 35mA, then you disconnect the cathodes of 2 of the 4 and now the B+ rises to 420V, but the -40VDC stays constant, the bias current will only raise by a few mA
If you plan accordingly, you can start on the cold side and land on the hot side without having to rebias.
At least, that's the way I see it.

[Willabe]

Using round numbers as an example:
If all 4 tubes are seeing 400V B+ and -40VDC bias voltage and they are running at 35mA, then you disconnect the cathodes of 2 of the 4 and now the B+ rises to 420V, but the -40VDC stays constant, the bias current will only raise by a few mA
If you plan accordingly, you can start on the cold side and land on the hot side without having to rebias.
At least, that's the way I see it.

Mmmmm, not sure that's right. Taking 2 power tube current draw off the PT/PSU from 4 tube load/current draw, I'm thinking that PSU dcv will rise more than 20vdc?   

If you use cathode bias, then you could just use 1 KR for each pair. The dcv voltage will still rise some when you disconnect a pair but it might just re-bias it self nicely if the dcv doesn't rise too much?     

[tubenit]

Once I figure out for sure about removing two tubes, I'll redraw this again.  Not sure what to do with this?

I've lowered the plate resistors on the LTPI to increase voltage.

I've essentially made an active FX loop using the cathode follower prior to the tone stack and then added a recovery stage.  This should help resolve PRR's suggested concern about the 9 volt preamp not having enough ummph to drive the 6L6's.  I used this same concept on several D'Mars ODS builds and it works quite well as an active FX, IMO.

Now the Mesa Subway bass preamp  ............ OR the LR Baggs acoustic guitar preamp can have another gain stage prior to the LTPI.

I've increased the PT to 300ma (assuming about 140ma per pair of 6L6's)

With respect, Tubenit

[SILVERGUN]

Using round numbers as an example:
If all 4 tubes are seeing 400V B+ and -40VDC bias voltage and they are running at 35mA, then you disconnect the cathodes of 2 of the 4 and now the B+ rises to 420V, but the -40VDC stays constant, the bias current will only raise by a few mA
If you plan accordingly, you can start on the cold side and land on the hot side without having to rebias.
At least, that's the way I see it.

Mmmmm, not sure that's right. Taking 2 power tube current draw off the PT/PSU from 4 tube load/current draw, I'm thinking that PSU dcv will rise more than 20vdc?   

If you use cathode bias, then you could just use 1 KR for each pair. The dcv voltage will still rise some when you disconnect a pair but it might just re-bias it self nicely if the dcv doesn't rise too much?   

Depends what his PT secondary current capacity will be
More capacity = less drop

[Willabe]

Depends what his PT secondary current capacity will be
More capacity = less drop

Yes, should be. You didn't bring that up before, unless I missed it? And under load a PT rated B+/current, it's dcv will go up, so you have to consider that also, might have to try a few PT's? Also higher current capacity will/should be a stiffer sounding amp, so maybe that will be a better thing, maybe not? 

[SILVERGUN]

I just had to know, so I ran some tests.
The test subject is my Silverface Bassman 100 that I converted to Bossman 100, but I always run it with only 2 power tubes in, so I should have renamed it the Bossman 50.
It's got Twin iron in it, basically the same PT that I suggested for T to use.
I had 4 old Sylvania 6L6GCs that were not a perfect match, but worked for this experiment.
Unloaded (no power tubes installed) B+ is 480VDC and bias voltage is -48VDC
With all 4 tubes installed the B+ dropped to 452V and the idle current from left to right was 36/29/39/37mA
So I pulled the high and low which happened to be the 2 center sockets (29/39)
With the remaining 2 tubes in, the B+ came up to 462V and the current was now 40/40mA.,
 
T, just as I have heard you tell others, I'm not trying to sway you either way, I just wanted to provide some data to back up my claim....and I wanted to see it for myself.
Check out the first result on this page for a nice, affordable PT that will yield similar results
 

[92Volts]

If the rise in B+ is significant with the transformers etc used, you could use a dual-pole switch. One pole switches out the cathodes of 2 tubes disabling them. The other pole introduces a resistor in the bias supply which drags the voltage slightly more negative. Or removes a resistor which drags it slightly more positive.

EDIT: If you use a second bias pot instead of a fixed pull-up resistor, it shouldn't even be hard to tweak this. Set the bias for the 2-tube position which should be more negative. That won't be affected by further tweaks because the second pot will never be in the circuit during this application. Switch in the extra tubes and second pot, and tweak until it's right in that position as well.

As always pots should be used to pull bias voltage up (towards ground) so if they lose connection or fry or something, the voltage defaults lower.

[DummyLoad]

Once I figure out for sure about removing two tubes, I'll redraw this again.  Not sure what to do with this?

I've lowered the plate resistors on the LTPI to increase voltage.

I've essentially made an active FX loop using the cathode follower prior to the tone stack and then added a recovery stage.  This should help resolve PRR's suggested concern about the 9 volt preamp not having enough ummph to drive the 6L6's.  I used this same concept on several D'Mars ODS builds and it works quite well as an active FX, IMO.

Now the Mesa Subway bass preamp  ............ OR the LR Baggs acoustic guitar preamp can have another gain stage prior to the LTPI.

I've increased the PT to 300ma (assuming about 140ma per pair of 6L6's)

With respect, Tubenit

buy one of these: doug's scratch and dent - he should have 2 left - i bought two of the other 3. they work admirably. no issues with the dented end bells, they don't impact the windings.


--pete

[tubenit]

Pete,

Thanks for the head's up on the trannies!

I think the next step is to meet with my friend and play his bass thru his Mesa Subway preamp into my (active) FX return jack into the LTPI and my power amp plugged into his bass speaker.  IF that sounds good ............. it may be a worthwhile project.   Will post results of this sometime hopefully in the near future after experimenting with this.

Appreciate all the advice and thoughts!  With respect, Tubenit