4 x KT88 Bass amp

[Smokebreak]

Finally I get to build one of these. Looks like it's gonna be the Model T 2nd Gen, or the "red knob" one, for a bass player(who has also stated he'll play guitar through it). I'll use KT88s.

I'm getting a parts list together and have a couple questions, as I've never played one of these amps, and don't know what they feel like.

Any reason why I shouldn't up the initial filtering? 20/20 seems low for any big amp, really, but for a bass amp?

What is the purpose of the 50r 5W plate resistors? Some crude fusing?

For the mid inductor, I'm unclear if I can use a $1 leaded inductor, as I did in an Ampeg build, or if I need to use something like the Triad I have listed below

Here's what I'm about to order, all from Mouser :

OT Hammond 1650TA 4/8/16 1K9 10lbs
PT Hammond 278CX 800VCT 12lbs
Inductor Triad C24X $10
Choke Hammond Manufacturing 159V 500mA x 1
Filament transformer 166S6 6.3VCT 10A

I drew out the mid-switch, at least how I think it's wired, and found a page with some mods that I haven't gone through yet. This build won't really kick for about 3 weeks or so, but I wanted to dump info as I get it. Mid switch is for 3pos rotary or SPST on/off/on

http://www.angelfire.com/blog/sunnmodelt/index.html

[Leo_Gnardo]

Built a nice fat sounding bass amp @ 19 years ago with similar iron. 278CX got me a nice stiff 520-530V B+. Used a Hammond 1650T output, what's the "A"? I don't see it on Antique/CE site where I'm used to looking. In any case this OT flummoxed me something awful until A/CE revealed the right way to hook up the secondary, I had to buy their special switch for @ $20 to get the literature, never did use the switch to this day. Now they treat customers a bit friendlier. Here's how the 1650T went, there's two secondary windings, one has 4 and 8 ohm taps. Another secondary has a 4 ohm tap. You're supposed to parallel the two 4 ohm taps for 4 & 8 ohm use, or wire the second winding in series with the first for 16 ohms. I gave up trying to include a 16 since that's a rarely found impedance for bass cabs.

Plate "fuse" resistors I'm sure you could leave out unless you really feel like having them, and you could go the SVT route with smaller value resistors if you do.

I used a pair of 220 uF series caps for main and SG filtering. Yes 20 uF of reservoir is puny for an amp this size unless you like farty tones along the lines of Felix Pappalardi. There's some charm to that I s'pose & luv luv luv listening to Felix's tone & bass lines but it's not for me, I need cleaner than that.

I borrowed preamp ideas from Fender's white/tan early 60's Bassman, added an ordinary mid by varying the "tail" resistor on the EQ string, no inductor at all. Just 10K mid control as I find it a good idea to have a dip at the mud frequencies 100-300 Hz.

The owner of the amp's been very happy with it & played hundreds if not thousands of blues gigs since 1996. Used GE 6550A until the supply started to get super expensive, then JJ KT88. In all that time he's on his third set of output tubes. Sounds lush & plush, just the way it's s'posed to. When fans ask him "where's the clank?" he answers "it's a BASS, not a TREBLE."

[Smokebreak]

Thanks Leo, I'm gonna check with him and see just how tight he likes it.
The TA is the easy wire secondaries @ 4/8/16. I stared at the A for a bit, had it figured out, then was like where am I gonna get that switch?!?

I ordered the JJ KT88s, too, so glad to hear your vote of confidence.

[Leo_Gnardo]

Thanks Leo, I'm gonna check with him and see just how tight he likes it.
The TA is the easy wire secondaries @ 4/8/16. I stared at the A for a bit, had it figured out, then was like where am I gonna get that switch?!?

I ordered the JJ KT88s, too, so glad to hear your vote of confidence.

Good thing Hammond did that, the other way was mind boggling, why they ever did that the hard way? And why the TA version isn't listed by A/CE? 'Tis a puzzlement. But they do have that special switch kit... feh. Go with the TA version nice to know Mouser's got 'em. In fact it was a bit of susprize to find tube-compatible transformers in the Mouser catalog at all a couple years ago.

[JazzGuitarGimp]

I don't know if this applies to tubes, but in SS amps with paralleled output transistors, it is customary to add a small resistor in the emitter leg of each output transistor. It's usually a fraction of an ohm, and its purpose is to balance the current between the transistors. Without these resistors, one transistor tends to want to do all the work, which of course it can't (otherwise, the output section would not employ paralleled transistors). Eventually, this leads to the destruction of the output transistors. I have no experience with high power tube amps, but from what I have read, a high-powered amp (I'm guessing you'll be shooting for about 200W) of this nature tends to be squirrelly in the output section. I am guessing that those 50R resistors might serve another purpose, other than fusing.

[Phil_S]

I'm not sure if this is the same problem I encountered with a Hammond 1635. It's still in the box, but I'm ready to use it. I bought a suitable rotary switch on eBay for not so much money. IIRC, someone here supplied a wiring diagram. I couldn't wrap my mind around it.

[Leo_Gnardo]

I'm not sure if this is the same problem I encountered with a Hammond 1635. It's still in the box, but I'm ready to use it. I bought a suitable rotary switch on eBay for not so much money. IIRC, someone here supplied a wiring diagram. I couldn't wrap my mind around it.

Solid color lines are the 0/4/8 winding, with white stripe the second 0/4 winding. In 4 and 8 positions, the 0 and 4 of both windings are paralleled. In the 16 position the 0 of the second winding is connected to 4 of the first, and the green stripe wire, 4 ohm on the second winding, becomes the 16 tap. OT secondaries don't add up arithmetically, two 4's in series yield 16. Looks like the same switch A/CE sells.

What worries me is the current capability of the switch contacts, they look light duty on the one that came from A/CE. I've even seen the Marshall rotary switches which are supposedly heavy-duty, badly frazzled inside. That's why I eventually went 4/8 only, and a heavy duty DPDT toggle with contacts paralleled for presumably 30 amps capability. With a rating like that it should last near forever. At minimum you'd need a switch with 5 amp contacts for a 200W amp and that's a blue sky sunny day "if nothing else goes wrong" figure right from P = I squared x R. Those rotary switch contacts look awful dinky to handle 5 amps.

[Phil_S]

Leo, I believe I misunderstood what is adequate. I hear you saying this one won't do. (I know it is 4 positions, but can be made to work. I think you call this 3P4T.)

At one point I solved this problem with a two toggle switches, but decided there was to much room for user error. I suppose it isn't awful if you plug in the wrong load

[Leo_Gnardo]

The switch in your photo looks a bit pokier than the little thing Antique sold me in '96. If you could find the manuf's specs on it and they claim 5A or more then I wouldn't be so worried. Also if you're using the amp yourself, you're bound to be more careful no doubt than some half-blitzed roadie plugging & unplugging cabs while someone's playing the amp at full tilt. And I've seen that done... and amps wrecked thereby. When everything's under control, no worries. When the amp's temporarily - we hope - trying to drive a short circuit, or off-spec loads like 2 ohms, that's what eats up switch contacts. I thought of the 2 toggle solution & turned away from it like you. I'd say don't bother with 16 ohm setup unless you're sure it's going to be called for, then you can use a heavy duty toggle or leave out the switch entirely and have separate 4 and 8 ohm output jacks.

I've used the HD toggle solution in old SVT's too, not trusting the dinky contacts on the extension cab jack switch to handle 300+ watts to 2 ohms, that's more than 12 amps of current at 2, about 9 amps at 4 ohms. Leaf switch, naaaah, gimme a big Carling toggle.

For smaller amps say 75W on down the rotary switches don't scare me so much.

[Phil_S]

I wish I knew what I have! That's why I showed the numbers, hoping someone can identify it. 228-7314-2-813. 228 is Lenz wire. April 1973? LOL, good luck with this one! https://www.etsy.com/listing/229819059/ ... g-co-radio

I can tell you it's heavy duty for a rotary switch. 5A, goodness, I guess it needs to be field tested to see if it holds up!

I have one 16 ohm cab I like, loaded with a ceramic Weber Blue Dog and probably the only speaker I've got that is higher wattage rated. So much for simple, huh?

You make an interesting point about private use. You know, I'm just here in isolation. If not for TAG and another place or two like it, I'd be totally cut off from human interaction on electronics-related matters. Do you mean to tell me that people are so uninitiated about care of their gear that someone might switch the amp while it's hot? Cripes! That's a scary thought.

Apologies to the OP for this hijack.

[Smokebreak]

No worries at all Phil, it's all related in my book, and besides, we all need an outlet to talk about our switches;) I got some NOS rotaries a while back that just "feel" like they can handle anything. That's very unscientific, I know, but sometimes you gotta go with your gut.

I've been reading about some of the mods and have an amended schematic for my upcoming build.
-One of the MV gangs goes away(the one that attenuated signal before the 2nd stage).
-Apparently the mid boost is annoying to some, as even at "0", it's still a modest boost, so a large cap is added from the unused end of the midpot to the cathode, to eliminate any boost, as the large cap swaps the LC network. It also acts as a variable cathode bypass, with the 25K put under it, so I can experiment with diffent caps, depending on what the amp needs, at that point. It's better explained in the link, but neat stuff.

No luck on a 25K RA pot(at an acceptable price), but I'm not even sure why the presence would need to be reverse audio taper anyways. Maybe they had a ton, along with the linear 1Ms

[Stevem]

Yes, using no more than a 10 ohm resistor in the cathode circuit of a duel or more output tube will help the tubes share the load so to speak , but a bias balance adjust is a far better way to go.
If you really want to go top notch then install a tube or SS regulator circuit for the amps screen supply, as its really that voltage sets the operation point of the tube.

[martin manning]

No luck on a 25K RA pot(at an acceptable price), but I'm not even sure why the presence would need to be reverse audio taper anyways. Maybe they had a ton, along with the linear 1Ms

You've no doubt noticed that the action of most presence controls is all in the last bit of the rotation. Use of a RA pot will spread it out.

[Leo_Gnardo]

No luck on a 25K RA pot(at an acceptable price), but I'm not even sure why the presence would need to be reverse audio taper anyways. Maybe they had a ton, along with the linear 1Ms

You've no doubt noticed that the action of most presence controls is all in the last bit of the rotation. Use of a RA pot will spread it out.

And you can "fake" an RA pot by starting with a linear one & bridging wiper to "hot" terminal with a resistor. You wind up with an overall value less than the original pot's value, but you can "bend the curve" to your liking. Start out with say a 50K or 100K linear & swap resistors until your ears are satisfied.

[martin manning]

Edit- A RA pot can't be faked this way. See below.