35 watt 1986 bass, comments please

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music321
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35 watt 1986 bass, comments please

Post by music321 »

Hi,

This is my first posting here. I have in mind a build I'm thinking of doing. I have basic circuitry knowledge, but have gaps in my knowledge regarding amps. An electrical engineer will be helping me with this. Translating schematics into layouts is a pain for me, but the EE can help me out. If I have both a schematic and a layout, I can easily correlate one to the other. Trying to put together a layout from just a schematic is where I have difficulty.

Essentially, I want to build a Marshall 1986 with lower volume, comparable to that of a JTM45 (35 watts or so). I want it to run el34s (if appropriate), have a lighted power switch, a shared cathode, volume controls for each preamp, some way to control volume post-power amp (either power scaling or master volume), a spring reverb that can be switched in and out, NO standby switch, an effects loop, solid state rectifier, external bias adjustment, and MAYBE have a line out and a switchable "sag simulator" involving a resistor and choke on the power supply. These last two things will be left out if they present significant headache.

So, a few questions if you don't mind.
To me, this seems pretty straight forward since the JTM45 and the 1986 are very similar. Would you agree? I'm sure this sort of thing has been discussed here before. If there's a thread (I did look, but didn't find anything), please point me in the right direction. I wonder if it's even worth modifying the 1986 circuit, since I'd only be doing so to bring down the volume. Do you think that doing so is even necessary/desirable, since I'll have control over the overall volume? The only real advantage to doing so (aside from volume reduction "tone issues") would be a possible weight savings. If this mod is desirable, I'm thinking that I'd want to either start with a JTM45 layout and modify it to make it more similar to the 1986. Is this right, or would I want to start with the 1986 layout, and modify it. Does it matter? If nothing else, I'll start with the layout presented on the Ceriatone website, unless someone has a better layout. I'll be using a pedal in the loop, so do you think the loop should be buffered? I've seen some loops pre-built as modules. Is this the way to go? If I can in fact use el34s, I'm guessing that I'd have the same internal voltages as the 1986, right? Finally, I came across this transformer pair on ebay. Do you think it would be appropriate for my purposes? There's no indication of windings, etc. However, it's meant for 6L6 tubes. The price seems right. If I can save $100 on transformers, so much the better.

Here's the link: https://www.ebay.com/itm/Vintage-6L6-Tu ... 1438.l2649

Thanks, I appreciate it. I've heard great things about this forum from TGP.
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pompeiisneaks
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Re: 35 watt 1986 bass, comments please

Post by pompeiisneaks »

Honestly, it sounds like you want 'Amp A but with less volume' in which case just adding a PPIMV will get you likely what you're looking for. Unless the tone you want is partly related to power amp distortion of course, and then the PPIMV will mess that up. Adding one isn't usually super hard. You just go replace the common 2 220k or similar resistors to ground on the area between the PI and the power tubes with a 250k dual ganged pot, and add a couple 2.2M resistors between them and ground as a 'protection' in case the wipers go out on the pots. It's the typical LarMar PPIMV that was explained by Ken Fisher in his trainwreck pages. You can read them on rob robinette's site here: https://robrobinette.com/The_Trainwreck_Pages.htm

Basically, you'd need to figure out the exact values to be sure, but you can just add a master volume to the amp that you know already exists and get what you're looking for, the ability to turn the volume down after the preamp is already at max doing what you want.

Then you can use an existing layout and schematic with a very minor modification.

~Phil
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music321
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Re: 35 watt 1986 bass, comments please

Post by music321 »

Thanks for the reply. It's funny, when I first started thinking about this, I wanted all sorts of bells and whistles. Now I'm willing to simplify. As for distortion, I don't need power amp distortion to be part of my overall sound. The PPIMV will be fine.
Last edited by music321 on Wed Dec 27, 2017 6:49 am, edited 1 time in total.
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martin manning
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Re: 35 watt 1986 bass, comments please

Post by martin manning »

PPIMV will alter the negative feedback ratio, so you might want to provide for some adjustment of the feedback resistor value to dial it in for different MV settings. 1986 and 1987 use the same basic layout, and adding a PPIMV isn't hard. There are some very good 1987 layouts around, and many examples of adding an MV. This recent thread has a couple: http://ampgarage.com/forum/viewtopic.php?f=9&t=30793 , and gut shots of 1986 and 1987. Power scaling has its advocates, but it is more complex and less reliable, IMO. You're adding more complexity with the loop, but there are some nice pre-assembled or kit loop modules available (see Metropoulos, e.g.). If you want Marshall tone, I would stick to Marshal clone transformers. You can spend a lot for Marstran, Mercury, or other boutique brands, but Magnetic components (Classic Tone) sound good and are quite reasonably priced. Lots of parts for early Marshall amp circuits available here, including boards and chassis: http://valvestorm.com
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Littlewyan
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Re: 35 watt 1986 bass, comments please

Post by Littlewyan »

Yes definitely consider the negative feedback as this I believe keeps the bass under control in the 1986 amp. Unless of course you're building the version that had very little negative feedback in which case you'll probably be fine.
music321
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Re: 35 watt 1986 bass, comments please

Post by music321 »

Why is building a 35 watt 1986 not as good a solution as building a 50 watt head? Is it because more modifications will have to be made? Is it because there will be a loss of tone with smaller iron?

Also, I see that Ceriatone makes a model called "British Style 36 watt" which uses El84 power tubes, and is essentially a higher-wattage model 1974. Might I just want to modify this design, or am I better off starting with the Marshall schematic?

Thanks.

Link (http://www.ceriatone.com/ceriatone/wp-c ... iatone.jpg)
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xtian
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Re: 35 watt 1986 bass, comments please

Post by xtian »

All things being equal (circuit, iron, tubes, voltages), you will find it nearly impossible to tell the difference between 50 and 35 watts.

Four EL84s sounds very different, in my experience, to a pair of EL34, which I far prefer.

Last thing I want to add is that 50-100 watts is wonderful for cleans. The best cleans in the house are my 100 watt ODS and 80 watt Fender Twin.
I build and repair tube amps. http://amps.monkeymatic.com
music321
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Re: 35 watt 1986 bass, comments please

Post by music321 »

Thought I'd make sure before I get too committed to any particular plan. I'll stick with the 50 watt.
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martin manning
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Re: 35 watt 1986 bass, comments please

Post by martin manning »

music321 wrote: Tue Dec 26, 2017 6:48 pm...a spring reverb that can be switched in and out...
I didn't notice this among your list of desired features above. This is a significant addition, and will require extensive layout changes. Since you are adding an FX loop, why not use an outboard reverb? Keeping it simple is a good idea. I also had a look at the e-Bay transformer set linked above. Those are nothing like what you want (OT is too small, PT voltage is unknown), and I'll repeat my recommendation to stay with Marshall clone parts. If you are looking for cheap, a DIY amp will not necessarily achieve that goal. Certainly you can build a nice amp for less than the cost of a vintage 1986, but I'd have a budget of at least $1,000.
music321
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Re: 35 watt 1986 bass, comments please

Post by music321 »

That's good to know regarding the reverb. Regarding the loop, ceriatone makes one (http://www.ceriatone.com/mod-buffered-loop-switch/). Would you recommend this choice, or going in another direction?
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martin manning
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Re: 35 watt 1986 bass, comments please

Post by martin manning »

I like this one better... much neater! https://store.metropoulos.net/products/ ... x-loop-kit
music321
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Re: 35 watt 1986 bass, comments please

Post by music321 »

looks good. I like the installation instructions.
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