JCM 800 1992 100 Watt Bass Amp Clone

[Paperweight]

I'm in the process of building a copy of the above mentioned amp and have a few questions if anyone can answer them. I've built almost 10 guitar amps by the book but this will be my first bass amp. I kind of wanted to add a little bit of beefiness to the original design.

I noticed on the schematic that the entire preamp section from input to phase inverter has only one 100uF filter cap and thought that was strange. Does anyone have any idea why they didn't decouple the sections like on almost every other design? I'm using a 100 watt Marshall chassis that has 6 holes for can type power supply caps. Would it hurt anything to go ahead and use the can cap over the preamp board to decouple the stages like on other Marshall designs?

I got ahead of myself scheming on what transformers I wanted for this build. I have a quad of Electro Harmonix KT-88s so I decided to get a 100 watt output transformer from Mercury that is wound specifically for 6550/KT-88s. I then had the idea of getting the power transformer from Mercury that offers a B+ of 560 volts. I was thinking along the lines of more headroom and maybe a little more power. Please tell me if I screwed up here. I know I'll have to tweek the resistor value to get the B+ down to a manageable level for the phase inverter.

Lastly, I thought about increasing the filtering for the plates and screens. The plates will have 275uF @ 770 volts and the screens will have 125uf @ 770 volts. Between the plates and screens will be a 29 Henry choke also from Mercury. Is this too much filtering for a bass guitar amp?

Any thoughts, input and help would be appreciated. Thanks.

[jhadhar65]

>Does anyone have any idea why they didn't decouple the sections like on almost every other design?

I'll guess that since bass preamps aren't usually gain monsters (in the Musician's Friend sense of the phrase), Marshall could get by with less decoupling without increased risk of ill effects.  A penny saved is a million earned.

>Would it hurt anything to go ahead and use the can cap over the preamp board to decouple the stages like on other Marshall designs?

Nope, as long as you give room for power tube dissipation.  They generate a lot of heat, which caps don't like.  Some people don't like cans.  They suit me fine.

>I was thinking along the lines of more headroom and maybe a little more power.

Headroom maybe, but depends on other factors, too.  It will make no practical difference to power output and perceived loudness.  Within limits, you may be better off with lower B+.  I'm going with you here trusting that 560V is in fact potential B+ and not HT, but your cap ratings of 770V make be wonder.  Are you sure those are 'uF'?

>Is this too much filtering for a bass guitar amp?

Not necessarily.  Bass amps can often benefit from additional filtering, which can help tighten up the low end.  After some subjective point, you may decide that the amp has become too sterile.  Look at other designs and see what they did, especially if you're familiar with how those amps sound/play.  Try it their way to start.  If you build your board or arrange the internal layout of your chassis properly, you can add additional filtering later and see if it fits your sonic goals.  Always plan to experiment past the initial design build.  After 10 amps, you know you'll experiment anyway.  Planning for it up front may make it a little easier later on.

[Paperweight]

Thanks for your very informative reply.

I'm using 4 JJ can caps for the plates and screens. 2 of them are 500uF @ 385 volts (in series) for the plates. The other 2 are 250uF @ 385 volts (also in series) for the screens. Is 770 volts high enough for the turn on spike?

What I meant to say is that the hole for the preamp can cap is over the part of the circuit board for the preamp circuitry. It's the farthest away from the power tubes so it won't receive any abuse from the heat.

I'm assuming the final voltage under load after rectification and filtering is supposed to be 560 volts. Some of the early Marshalls had power transformers that gave this exact B+ on the schematics. A pair of KT-88s could handle this easily I assume. What benefits would lower B+ offer an amp that uses 6550s/KT-88s only?

Yeah, I often tell people that twice as much power only amounts to 3 more decibels and they will hardly notice the difference. They often don't seem to be able to grasp that. I'm building this amp for a guy that currently uses an Ampeg SVT Classic. He has to use 2 large 8 ohm cabinets with it at all times. Every show he plays, he has to lug this massive rig around and set it up. Meanwhile, the guitarist only uses a 50 watt head and everything on stage gets miked anyway. He can use this amp with his 4 X 10 bass cab and save himself a lot of trouble.

Once again, thanks for your response.

[bigdaddy]

Why a 100 watt output tranny? 4 of those tubes should do 120 or 150 easily. Even 200 watts in the old days. Marshall Majors had 4 KT-88s and were 200watts, I'm pretty sure that's what Jack Bruce was using with 2/4-15 cabs.

My thinking is that higher voltages will deliver the most power if you want to use it. Using a 100watt OT is not what would call using those tubes to there best operation. The 100 watt would be more toward a quad of 6L6GC or EL34 or even these new JJ KT77 which take a very high voltage.

Just my opinion.

[Paperweight]

I have an old set of 6550s and a fairly new set of KT-88s that have seen little use in my stereo. I figured why not use them. They will last forever in this design and will be pretty clean sounding. I may experiment with using JAN Philips 6BG6 tubes since I have a master case of them and they are built incredibly tough and will handle anything you throw at them. Since they have a plate cap on top, they will handle a B+ of about 700 volts. They cost me $3.25 each and I have about 90 of them left. They have characteristics pretty close to a Philips 6L6.

I could add an extra driver stage after the phase splitter if I wanted the full use of the power tubes. The 200 watt transformers cost over $400 each at Mercury. Since this is my first bass amp, I wanted to start out conservatively. Besides, the chassis I'm using is .090" aluminum. It's just sturdy enough for the 100 watt transformers. If this goes well, next time I'll build a more powerful bass amp that uses the 200 watt Sunn output transformer. From what I've heard, it's a very nice hi-fi chunk of  iron that goes pretty low.

The whole point of this build was to be a smaller, lighter replacement for an Ampeg SVT Classic and the 2 cabs needed to run it. With a 100 watt bass head, the bassist will only need his 200 watt 4 X 10 cab. Every venue they play at, the whole band gets miked anyway. These are medium sized bars at best. I don't think the SVT has ever been close to being turned all the way up.

[bigdaddy]

Why not try using a pair of 6550s instead. What is your primary? If you have a multi tap secondary you can easily change to 2 power tubes. depending on you speaker cabs. If you're miked the 70 or so watts from a pair of 6550 going through the 4x10 cab should be fine. Unless it's not a really high quality cab, if it's like an Eden XLT then you're good to go. Some of those 4x10 cab cannot take high wattage and with that smallish OT and 4 huge bottles it sounds like a recipe for a disaster, a mismatch of things.

When it comes to bass, I know this I built bass rigs and small to medium sized sound reinforcement systems, the efficiency of the speaker cabinets and the XmaX of the speakers are the most important things in a bass rig. It's not about overall wattage, it's about being able to reproduce those long sine waves and have them be able to be heard in the back of the room without rumbling the whole building. Since you're all miked, I'm sure he's using a direct box so his amp is probably being used for a little stage volume and more of a monitor for his bass. Clean low notes without an over abundance of overtones and speaker distortion is were it's at. That will make your sound smooth and pleasant to the ears. The natural compression from the tube amp should give you that with a good speaker system. By playing harder you should get that growl some bass players like, that will naturally drive the amp.

The SVT with the 8-10 cab sounded great for about 20 feet max and then died. The Acoustic 360s had the throw with those ported 18" cabs. A lot of guys went with a pair of SVTs one with 2 8x10 cab and one with 2 V4B 2-15 cabs. During the 80s when the sound systems started getting really huge, they wanted the stage volume to drop for the monitors.

[Paperweight]

Not sure about the primary, Mercury never lists those things. I can check it sometime with a signal generator and an ohmmeter.

I thought about the 2 output tubes for 100 watts approach. Wouldn't that require driving them harder than the stock design? The signal swing going to the output tubes would have to be increased somehow. Driving 2 tubes to 100 watts would take the same amount of drive as pushing 4 to 200 watts. The Marshall Major used another preamp tube post phase splitter to acheive that. What's wrong with a quad of tubes that's not being pushed too hard? I'd assume the power transformer would sag long before the amp could even think about producing 200 watts. That and the lack of real drive should keep the tubes inside the limit of the 100 watt output transformer. EL-34s, being true pentodes, are easier to drive than beam-power tubes. They could probably produce more power in this amp than KT-88s. I might be talking out my can on this but Menno van der Veen had a super-pentode amp where a pair of EL-34s produced a fairly clean 90 watts. Anyway, someone mentioned to me that even with KT-88s and a higher B+, you aren't going to get anymore power than the stock design.

I asked the bass player to double check the rating of his 4 X 10 bass cab. With his current setup, the power from the SVT is split between it and an 18" woofer. I'm just building the amp to make life a bit easier for him. He can deal with the speakers. Then again, Bill Fitzmaurice had some really neat horn designs that possibly included a bass horn if I remember correctly. Too bad I'm not a woodworker.

[bigdaddy]

I think I saw those speaker cabs, really high SPL. More like the old time horn designs like Perkins bins and the voice of the theater types but rear loaded. Great throw but they can get honky sounding if not EQd correctly.

I just noticed that small iron for OTs really causes a lot of compression and that's great for guitar. So many great guitar amps have smallish OTs like the deluxe reverb. For trying to reproduce all those frequencies below 80 hz is seems to me that the more Iron the better the lows. So it's not really completely about the tubes, it's about reproducing lows. 4x 6550/KT-88 are probably going to produce way too much no matter how you bias them and I just don't think that OT can handle it. I could be totally wrong......plus that 4x10 cab could be an issue too. If the cab bottoms out or the speaker over extrude you'll get mud, plus I bet that cab has a low SPL and requires a lot of wattage to drive.

In my opinion I would try the 4xKT-88/6550, 2xKT-88/6550 and maybe 4x the new JJ KT-77s or 6L6GCs. Also I know it looks cool and sounds cool having a Marshall but their bass amps really sucked, bad design for a bass preamp. If I were trying to design a really good sounding bass amp I would use either an Ampeg B-15 or fender Bassman and just build it a bit better and with more iron and tubes. Like a 2x6550 B-15 or a 4x6L6GC BF bassman. Those original amps through either an Altec or JBL D-140 kept up with the band pretty well until the guitar players started using bigger amps and turning it to 10 or 11....

At one time the B-15 ruled, great bass sound and lows from a 30 watt amp. Many guys used Dual Showmans and cabs with D-140s for bass too. My experience was that the Marshall bass amps just did not cut it except for the old majors which weighted a lot and blew up a lot. A 50 watt bass amp with a speaker cab of 110db spl is really loud. It's all about the sum of the parts.

Call Paul at Mercury he'll tell you the specs. Good luck!!!!

In all honesty his best bet is to get a tube mic preamp like a solo 610(which can also be used as a direct box) DBX 166/rane or something EQ and a solid state power unit or amp in a rack like an old lower wattage SS crown/crest/whatever or even an old GK RB800 and have you bullet proof the RB800 and improve some of the components. Of this I speak from experience, I did put bass rigs together.

[FYL]

I may experiment with using JAN Philips 6BG6 tubes since I have a master case of them and they are built incredibly tough and will handle anything you throw at them. Since they have a plate cap on top, they will handle a B+ of about 700 volts. They cost me $3.25 each and I have about 90 of them left. They have characteristics pretty close to a Philips 6L6.

The Sylvania / Philips ECG 6BG6GA are 7581A's with a plate cap. Same structure, same everything: Philips ECG used the same guts for their late production 6L6WGB's, 6L6GC's, 7027A's and 7581A's. Treat them as 6L6GC's with 35-watt plates and they'll be fine.

[Paperweight]

I'll see what happens with this output transformer. Since I already bought it, I'm stuck with it until I can cough up another couple hundred bucks if it's crappy. I didn't expect or really want low bass. I was thinking if it rolls off the fundammentals, you'll end up with the kind of bass you can hear instead of feel. Anyway, if all else fails, I'll just rewire the preamp to something that works. There's tons of room to work with in a 100 watt chassis. This guy isn't really picky. He barely knows how to play bass and doesn't know how to set the controls on the Ampeg he uses. If it makes some bass like noise, he'll be happy.

I bought the Philips 6BG6s from SND Tube Sales in 2003 when he was strapped for cash and sold cases for $300 and $25 shipping. He has a write up on his site explaining all about them. Went through them and tested their plate current draw in a guitar amp. Picked out several matched sets and built a Weber Java guitar amp with one set. Kid in a local band uses it now. They're a great tube to tinker with. Cheap as dirt and sound pretty good too. Had a set in my stereo for a while.

[Paperweight]

One of the original choices I had in mind for an output transformer was a Hammond 1650T. 30-30kHz, 14 pounds, 120 watts, and the price isn't all that bad.

Which Ampeg had the flip up top? Was it the B-15? I know one of them was used in lots of Motown recordings. I read the Ampeg book when I first got it in '99. That would be a great project. We'll see what happens with this design first.

[bigdaddy]

They had a B-12, B-15 and a B-18, all were flip tops. The only time I saw a B-18 was the ex bass player from the Thunderbirds from Texas and his new band, that was like mid 1980s. They came to New York, the amp did not produce enough volume for the room and the sound was muddy. I forgot the name of the bar but it was in Manhattan and had a big Iguana or something on the roof.

Yes the B-15 was the Motown amp. The Stax bass amp was,...........A tweed twin with one speaker, the other one was blown....

From the way you talk about the bass player you're making the amp for he is better off with an RB400 GK and call it a day. One of the best sounding SS amps for bass ever made and cheap. The older ones sound terrific and can be had for less than $200 on fleabay. The only issue is the power supply, from what I understand the rectifier has issues. I always wanted to see if I could bullet proof one and then see how they sounded.

[HotBluePlates]

Make sure you look hard at screen supplies for any/all example bass amps that have 6550/KT88/6146 and the high B+ you're planning. Without checking, I think a lot of them were significantly below the plate voltage. It help the output tube make big power (high plate voltage) while not going over dissipation limits (lower plate current due to lower screen voltage).

This may be the difficult aspect of the build.

[dynaman1]

I'm in the same boat in that I'm also building a bass amp and wish to use up some of the higher voltage PTs I have laying around. I'm looking at 550VDC so I'm prolly sticking with KT88s. While digging through my stash of iron, I lucked out and found an old Halldorson OT that's rated at 75W. It's the same size as the average Marshall 100W OT so I expect it to perform well.

My bass player uses a Traynor YBA-1 and in spite of its Little OT and EL34s, it still moves a lot of air when pushing a huge Ampeg cab. I decided to use the same circuit and modify it for transformers/voltages. For the heck of it, I'm also going to add a MV.

Incidentally, I also came across an Peavey 100W OT that has UL taps. Good candidate for a Sunn 200 or similar. Give a holler if you're interested.

[Paperweight]

That reminds me, I still have the guts of a Fender 140 output transformer sans endbells. The whole amp looked like it had water damage so the laminates of both transformers were pretty rusty. I replaced it back before I knew better and come to find out later it was fine all along. Anyway, this one had ultra-linear taps too. Not a very popular amp and was only made in 1980. Last I heard, the guy that owns it doesn't even play guitar anymore. Shame too, it was a good clean amp.

The band's drummer has a really cheap solid state bass amp. If it hadn't fried the last speaker he connected it to, I'd tell him to give it to the bass player lol.

[Paperweight]

The reason I give the bass player a hard time is all the damage he does to equipment. It seems to be his habit of stepping all over the cables. I've repaired several mic cables and the input to the Ampeg after he sprung that stepping in the instrument cable going into it. He's big dude that repairs cars for a living. Anything else, he knows nothing about.

[PRR]

> the above mentioned amp

This plan? http://www.schematicheaven.com/marshallamps/jcm800_superbass_100w_1992.pdf

100uFd is very generous decoupling.

V3 is push-pull and ideally needs no decoupling.

Both sections V2 work near unity-gain.

V1B works at low gain.

I believe it will work. (And obviously it does.)

If you want to, put another 10K+100uFd between V3 feed and V2 V1 feed.

> a 100 watt output transformer ... wound specifically for 6550/KT-88s

At what voltage??

The way I read the plan which I linked above, the amp was designed around EL34.

You need four EL34 to make a 100W-120W amp you can sell to customers.

(There IS a 100W/pair condition on one EL34 datasheet but it requires strict regulation to stay within rated limits.... not applicable to a stage-amp.)

Two KT88/6550 will comfortably make 100W at the right voltage and load impedance.

It may be more robust and solid with four KT88/6550 working with the four-EL34 100W-120W conditions.

Yes, you could get more, but you already bought the output transformer. And for Deep Bass, you do NOT want to over-volt the OT. It won't roll-off, it will muddy-up.

The linked plan does not show nominal voltages so I cna't comment.

I do not understand how you are computing main cap values. The linked plan appears to show two 125uFd in series for 62uFd of main filter cap. I think this is adequate, not excessive. You could go two 220uFd in series.

If you honestly want to spend 27H for screen filtering, there is NO need to increase that. With the two series 100uFd or 50uFd screen cap you have 57dB ripple reduction, like 10 times more than most amps have.

> he has to lug this massive rig around

Get a one-18" box and a 400W transistor amp. Yes, tubes are better for the musician, but hell on the roadie, and leave a self-roadie-ing musician too tired to play at his best. And nobody else in the room knows the difference. Save the tube amps for rare intimate gatherings.

> This guy isn't really picky. He barely knows how to play bass

Then you are totally over-thinking. If he has too much money, build the Marshall as-stock using good generic parts. If he's poor, Banjo Center has new and used bass amps and will consider a credit-payment plan.

[Paperweight]

I started to really like the Reeves 225 bass amp and wonder how hard it would be to tweek a Hiwatt DR-103 clone for bass. I'm really starting to think the transformers I bought were a bad idea for this though. I was wondering if substituting 1 meg pots for the 500k used in a Hiwatt preamp would change the sound. I bought a bunch of 1 meg PEC 2 watt pots for the Marshall preamp and would still like to use them in this build.

So is it crucial to drop the voltage going to the screens of 6550/KT-88s even if they are not in an amp designed to operate in AB2? I figure it would take a 10 watt 2k ohm resistor to drop the B+ from 560 to say 460. The choke will drop it about 5 to 10 volts as well. Then the next resistor for the phase splitter won't have to be as large. Any thoughts on this would be helpful.