Noob - New tube bass amp project

[Garrett335]

Hey guys,

I recently had a friend contact me about building him a 100W tube bass amp.
I've never designed an amp myself as I am still a noob, and currently taking lessons with Gerald Weber. I have built several 20Watt plexi builds off of an existing circuit so I know how to work inside the amps etc.

I was hoping to maybe start a thread of my build as I'm going to have many questions and mistakes along the way.

Here is what I had in mind for the build.
SS Bridge Rec.
3 12ax7's, including a LTP, into 2 PP KT88's - (I've never installed a Fixed Bias before, could I do a Cathode Bias? or is that not a great idea with the amount of power this thing will be putting out? I'd need a huge resistor probably.)

Gain / Vol / Bass, mid, treble  (Possible mid cut or boost switches)
Eventually add a direct out XLR for tracking or club gigs, but right now I just want to get the basic circuit running.

He did mention that he'd like to get some crunch out of it; a mix between an Ampeg and an Orange.

Thanks so much for your time, I'll be working on the rough schem and upload, I'm sure it will have many errors lol.

PS. I was looking at these two transformers:
PT: http://www.classictone.net/40-18068.pdf
OT: http://www.classictone.net/40-18067.pdf

Many thanks!!!!

[PRR]

Never invent.

Especially for clients.

STEAL a known-good 100W bass-amp plan. Start from *transformers you can get* and see what they were used in.

IMHO: two new-made KT88 is marginally reliable for a 100W stage amp (and NO you can not make 100W/pair KT88 with cathode bias). I would plan for four.

Yes, this is a big amp, and you gotta ask: does he really want a 15-Watt bass amp tapped into a 400W Class-D module? Get some small crunch-power, then cleanly blow it up to fill a room.

[plumcrazyfx]

I'm interested to see how you progress.  I was thinking of building a B-15 with KT120s at one point.  I changed my direction when I was asked to convert a Acoustic 164 to a bass amp.  I'm currently planning a conversion to a 100W Ampeg B-15.  Wanted to stay away from cathode bias because of the 100W 4x 6L6GC so I started with the B-15NF schematic.  Also planning on going SS rectified and one channel and PI with 12AT7 because they are easier to find/cheaper than 6SL7s.  I'm going for a classic but more firm sound.  The Acoustic is planned for someone else but I was going to build a second for myself and go with off the shelf Hammond for the iron - maybe a Twin Reverb replacement output (the Acoustic has Shumacher iron).

[MFowler]

You only need two 12AX7 preamp tubes and two or four output tubes depending on what type of tube you choose.


I started with the Fender Bassman 100 design and modified.


But I did go for 150 to 200 watt iron so I used very large Hammond transformers.


Mark

[92Volts]

For best reliability (especially with new tubes), keep screen voltage low, about 1/2 plate voltage if it's really high. This is reflected in KT88 datasheets, except ultralinear use which has its own issues. Also, it's not supported by the OT you're considering. Low screen voltage is used in tested designs like the SVT.

This isn't crazy complicated. It either requires another PT tap or a beefy regulator system, and can be designed or copied with a fairly low parts count if that's your goal. However, fixed bias isn't the biggest concern here!

With iron like a Hammond 1650T, because of its 1.9k primary impedance, you could get the power you want with B+ below 450v. This makes power supply cap selection easier, and is easier on tubes letting you run the screens at nearly the same voltage as plates. Basically, you can copy a 100W Marshall poweramp, or something similar. 4x6L6s would be needed, depending on the exact voltage and output you're aiming for.

[Garrett335]

Never invent.

IMHO: two new-made KT88 is marginally reliable for a 100W stage amp.

Yes, this is a big amp, and you gotta ask: does he really want a 15-Watt bass amp tapped into a 400W Class-D module? Get some small crunch-power, then cleanly blow it up to fill a room.

When you talk about reliability, do you mean they have a high fail rate? Or maybe the high plate voltages and current being drawn burns them up faster than other output tubes?
I'd love to build it with two tubes but if Four 6L6s is a better way to go I'll look into that.
I think I just understood your comment about the 15watt tapped to Class D, you're referring to my XLR idea? Maybe not a great plan ha.

@plumcrazyfx
Your project sounds awesome, yes I'm hoping to cut my teeth on this build we'll see how it goes.

@MFowler
I'll look into that Schem, did you use this as a bass amp or guitar? Did you tweak the tone shaping to get a more modern bass tone?

For best reliability (especially with new tubes), keep screen voltage low, about 1/2 plate voltage if it's really high. This is reflected in KT88 datasheets, except ultralinear use which has its own issues. Also, it's not supported by the OT you're considering. Low screen voltage is used in tested designs like the SVT.

This isn't crazy complicated. It either requires another PT tap or a beefy regulator system, and can be designed or copied with a fairly low parts count if that's your goal. However, fixed bias isn't the biggest concern here!

With iron like a Hammond 1650T, because of its 1.9k primary impedance, you could get the power you want with B+ below 450v. This makes power supply cap selection easier, and is easier on tubes letting you run the screens at nearly the same voltage as plates. Basically, you can copy a 100W Marshall poweramp, or something similar. 4x6L6s would be needed, depending on the exact voltage and output you're aiming for.

Thanks 92Volts, I'll look over the data sheets like you are talking about.
What did you mean by the OT not working? for this application, I was just looking at the Primary impedance which was 4k.
I appreciate all the input. and yes I am trying to limit the cost and parts if possible. I'd like it to be easy to maintain (Tube replacement)
Thank you for the Hammond recommendation, I'll take a look at this, and consider maybe using a quad 6L6 setup.

[92Volts]

Thanks 92Volts, I'll look over the data sheets like you are talking about.
What did you mean by the OT not working? for this application, I was just looking at the Primary impedance which was 4k.
I appreciate all the input. and yes I am trying to limit the cost and parts if possible. I'd like it to be easy to maintain (Tube replacement)
Thank you for the Hammond recommendation, I'll take a look at this, and consider maybe using a quad 6L6 setup.

Ultralinear is where screens are connected to the OPT-- and indirectly to the plates through it. As a result, when the tube (including screen) draws peak current, the screen also sees lower voltage which reduces power dissipation.

The extreme end of this spectrum is to connect the screens straight to the plates, aka "triode mode" but that reduces output power too much. So you need additional hookups on the OPT primary side, which the Classictone OPT doesn't have. Hammond 1650 series transformers do have them.

I'm not sure I'd recommend UL mode. I'm just pointing out that while the screen voltage rating seems high for many tubes, its not recommended to be at that limit without doing something to limit screen power. Often it's best to just provide them with a (fixed) lower voltage.

[jjasilli]

Building on the above replies - you can get 100W from 2X KT-88's in Push Pull; fixed bias.  The screens should be 300V.  That means a special PT.  There's no other good way to put 600V on the plates and 300V on the screens.  I rebuilt 3 such amps for home theater.  As PRR says, they eat power tubes.


UL is a good alternative and can produce 100W with 2 power tubes, per the tube charts.  But if you really want 100W, +1 to prior posts that 4 power tubes is preferable.

[Garrett335]

Building on the above replies - you can get 100W from 2X KT-88's in Push Pull; fixed bias.  The screens should be 300V.  That means a special PT.  There's no other good way to put 600V on the plates and 300V on the screens.  I rebuilt 3 such amps for home theater.  As PRR says, they eat power tubes.


UL is a good alternative and can produce 100W with 2 power tubes, per the tube charts.  But if you really want 100W, +1 to prior posts that 4 power tubes is preferable.

Thank you very much for the input, looks like I'll look into the rout of a 4 banger

Thank you @92Volts for the explanation, I had not learned about UL until now.

[92Volts]

Building on the above replies - you can get 100W from 2X KT-88's in Push Pull; fixed bias.  The screens should be 300V.  That means a special PT.  There's no other good way to put 600V on the plates and 300V on the screens.  I rebuilt 3 such amps for home theater.  As PRR says, they eat power tubes.

A center-tapped PT can be used with a FWB rectifier-- instead of being grounded, the center tap becomes the tap for the 1/2 voltage supply. Most tube amp transformers can be repurposed for 2x voltage, plus a screen tap.

The problems are: 
First, it must be rated for unusually high current at low voltage (in this example it would be a large transformer, designed for 300vDC, a voltage associated with smaller amps). 
Second, in this configuration the HV taps no longer swing negative relative to the grounded center tap-- instead they swing to ground relative to the positive center tap. This makes bias supplies more difficult and a separate bias tap or transformer would be helpful.

All this does not strictly require a "special" PT, but it certainly brings more factors into consideration and rules out many standard PTs.

[jjasilli]

The KT-88 is a great tube.  (At least one published amp guru thinks it's the only power tube we need!)


The 100W 2-bangers I referred to above are Stromberg-Carlson APH-1100 PA amps from the early 60's.  The manual with schematic is posted on in the Schematics Section.


Also, it occurs to me that a 2X KT88 amp might benefit from inrush current protection at turn-on -- EDIT: MOV; thermistor; or some fancy SS circuitry.  Not sure: i) how to implement that; ii) if it will substantially improve tube life.  Maybe someone can chime-in here.

[John]

You should, before anything else, make sure your friend knows how heavy 100 watts of tubes is. 

[jjasilli]

Ditto to that!

[PRR]

Ah, now I know where you came from on the other thread.

> 2X KT88 amp might benefit from inrush current protection

I used to have 2.5 of 2-ch 8-bottle amplifiers. 2*150 Watts per chassis (I goosed to 2*180W). The bottles were 8417 which was a 6550-class tube with high sensitivity, same line-current. They had worked several years at FULL power running projector motors. One side caught fire, which I traced to a joint which had NOT been factory soldered, just hung on by the crimp until tarnish set in. I worked the surviving 5 channels pretty hard in various concerts, sometimes on rough power.

I do not recall any inrush limiter. I tended to power on one at a time but I know the whole bank got bang-started a few times when I had helpers.

FWIW: I also up-sized the filter caps about 5X (how I got the extra 30W/ch).

8417 fans(?) will kill me to know I could buy good new 8417 locally for $13.

[jjasilli]

Ah, now I know where you came from on the other thread. > 2X KT88 amp might benefit from inrush current protection
Yes!


8417 fans(?) will kill me to know I could buy good new 8417 locally for $13. 

[Garrett335]

What do you think about this circuit, might be a cool one to copy.
I guess it's a 60w, but paired with the right cabinet could be cool.
The 6AN8 is a strange tube in there though. Looks like it is being used for a gain stage, and a P.I. (V2a and V2b)

You should, before anything else, make sure your friend knows how heavy 100 watts of tubes is. 

hahah, well he has an Ampeg SVT right now, and wants something smaller, something he can push into a little overdrive/clipping.
This 60W might be the ticket.

Building on the above replies - you can get 100W from 2X KT-88's in Push Pull; fixed bias.  The screens should be 300V.  That means a special PT.  There's no other good way to put 600V on the plates and 300V on the screens.  I rebuilt 3 such amps for home theater.  As PRR says, they eat power tubes.

A center-tapped PT can be used with a FWB rectifier-- instead of being grounded, the center tap becomes the tap for the 1/2 voltage supply. Most tube amp transformers can be repurposed for 2x voltage, plus a screen tap.

The problems are: 
First, it must be rated for unusually high current at low voltage (in this example it would be a large transformer, designed for 300vDC, a voltage associated with smaller amps). 
Second, in this configuration the HV taps no longer swing negative relative to the grounded center tap-- instead they swing to ground relative to the positive center tap. This makes bias supplies more difficult and a separate bias tap or transformer would be helpful.

All this does not strictly require a "special" PT, but it certainly brings more factors into consideration and rules out many standard PTs.

Thanks so much for this input 92Volts

[jjasilli]

Yes, it does not meet the 100W power goal.


The customer seems to want overdrive.  Personally I'm not a fan of split PI overdrive tone (this could draw hate mail form Princeton lovers - you know who you are!); something to be aware of. 

[smacbride]

I believe that ted weber has a kit for these, you might check it out, at least the layout drawing could help.

[Garrett335]

I believe that ted weber has a kit for these, you might check it out, at least the layout drawing could help.

Thanks @smacbride!
That looks like a really cool kit the 6S100, I wish there was someone demo'ing it online to check it out.
I'll run this by him, maybe I'll just build this :p

[92Volts]

Yes, it does not meet the 100W power goal.


The customer seems to want overdrive.  Personally I'm not a fan of split PI overdrive tone (this could draw hate mail form Princeton lovers - you know who you are!); something to be aware of.

Besides tone, split PIs have borderline output swing to drive some tubes. -55v bias voltage needs 110v peak-peak from both the anode and cathode to clip either output before PI clipping. That output circuit could do 100w with higher voltage but that approaches 2 150v p-p signals from the PI!

The 6AN8 triode is beefy and it's fed a lot of voltage so this might be possible, but I'd worry about stuff like the heater-cathode voltage limits when using it that way.

[MFowler]

TAD 150w bass kit pdf shows schematic and layout    http://www.tubeampdoctor.com/images/artikel/Full_Documentation_150WSB_140213.pdf


I followed a layout for a single channel Fender Bassman 100 somewhat and modified PI and added deep switch to tone section.