| Author: | BarnaciousC (registered user: 8 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 10:52 ( . ) |
Hello everyone first post and all. I have a Mullard 5-10 hi-fi mono block amplifier that I would like to convert to a guitar amp. The amp has a ef86 preamp directly coupled to 12ax7 PI that goes into a cathode biased el84 push pull power stage. This amp has a lot of history as it was one of the first el84 amps produced plus I bought it for £25, here are some pictures and background info on the amp plus the schematic I traced from the amp. [link] [link] [link] [link] [link] As you can see I have put some adjustment ideas on the schematic, suggestions, ideas and comments are very welcome and appreciated :D Cheers. |
| Author: | Al Taylor (registered user: 1632 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 12:06 ( . ) |
It's not letting me pull the images up due to an error, except for the image of the magazine description... |
| Author: | BarnaciousC (registered user: 8 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 12:19 ( . ) |
Fixed [link] [link] [link] [link] |
| Author: | Kursad K (registered user: 1308 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 13:15 ( . ) |
Looks like a 5E3 tone control would work fine in that circuit. [link] Kursad |
| Author: | Paul Fawcett (registered user: 2923 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 13:55 ( . ) |
On 03/18/2012 @ 10:52, BarnaciousC wrote :
: As you can see I have put some adjustment ideas on the schematic, suggestions, ideas and comments are very welcome and appreciated :D : Cheers. -- As you point out, it *is* a piece of history. I would encourage you to think twice before diving in. The world's full of guitar amps, why not restore it? That said, there's a few flaws with you plan. The most serious is that the cathode-coupled phase inverter is direct coupled, and its bias depends on the DC level from the plate of the EF86. You can't just slap a coupling cap or tone stack in there without providing an alternative means of biasing the phase inverter. Similarly, you can't just disconnect the ultralinear connection for the screens without providing an alternative means of powering the screens. Several of your other changes seems unnecessary. There's no harm in leaving the power tube grid stops at 4.7K - in fact larger grid stops are more in line with good modern practice. The EL84 also allows surprisingly high grid leak values in cathode bias, so the 820K values there should work fine, even if they aren't "typical" for guitar. At the input stage, it is now more usual to put the grid stopper directly on the pin, with the grid leak coming before. 68K is also rather more than you want at the input from the point of view of noise. Now, the EF86 has low input capacitance, so you do indeed need to roll off some high end to prevent RF from creeping in at the input. But you'll be better off doing it with a smaller resistor, and adding some hundreds of picofarads of capacitance from the grid to ground. |
| Author: | joe kerr (registered user: 1685 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 14:54 ( . ) |
It is a very nice looking amp so it would be a shame to strip it bare and trying to recycle the chassis etc can be a lot more work than just getting a new one. Maybe just "borrow" the transformers and build an 18watt. Cleans and Rock. The coupling caps would probably be Ok, the tubes might be still good but the electrolytic caps and resistors would be suspect. |
| Author: | trobbins (registered user: 915 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 15:44 ( . ) |
I had a similar dilemma with an almost equivalent kit amp from Australia. As a kit (which was common for Australian amps) my amp was much more an amateur affair, so I didn't face the same moral concern. I decided on an AC30 clone with some embellishments. [link] But imho your amp is much more a fine example of the age, and if I were restoring it I would take a somewhat different tack: eg. aim to keep the external view original - maybe put power switch in spare hole - maybe rewire octal to extend out to a satellite module with tone control and input jack (but don't have voltage rails on it). Then in time the amp could be easily returned to its original purpose if needed, which nowadays would be as a stereo pair with a common controls console - and with a much higher resale value (due to the hi-fi fraternity). Ciao, Tim |
| Author: | BarnaciousC (registered user: 8 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 16:07 ( . ) |
My idea was to turn this into a guitar amp with as little change as possible. I will have to give some thought as of what to do, its a tricky one. |
| Author: | Kursad K (registered user: 1308 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 16:14 ( . ) |
On 03/18/2012 @ 16:07, BarnaciousC wrote :
My idea was to turn this into a guitar amp with as little change as possible. I will have to give some thought as of what to do, its a tricky one. -- Find a way to fix the bias problem of the PI, as Paul says. You'll need that if you want to add a tonestack to there. Others (UL taps, for example) can wait.. UL taps have an effect on the frequency response of the amp, but that's not an urgent issue. Kursad |
| Author: | Stephen Keller (registered user: 5250 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 21:33 ( . ) |
On 03/18/2012 @ 16:07, BarnaciousC wrote :
My idea was to turn this into a guitar amp with as little change as possible. I will have to give some thought as of what to do, its a tricky one. -- For minimal change, why not stick a clean boost preamp and an EQ pedal in front of it? Stph |
| Author: | Celeste hall (registered user: 2235 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 16:42 ( . ) |
See if you can sell it for enough to buy an 18w IIb kit. If you have no use for a mono hifi amp, and it won't fetch half the cost of an equivalent guitar amp kit, then...... If you have to have a classic tone stack, the PI bias issue in not a major problem to over come, just a few wires moved and an extra resistor or two. There are however other tone controls topologies that will not require you convert from DC to AC coupling of the PI. The garnet is a 1 knob that will work with the DC coupling, just some changes around the EF's cathode. If you need more controls, you can split the controls into different parts of the circuit. If you search Melissa here, it is a good example of putting each control where it works the best rather then in a big lump. Unless you know you will not like the UL, there is no reason to disconnect it. Ul got a bad name with guitar amps with amps where it was added as an after thought to amps that were already using a lot of global NFB. Your amp only had UL for NFB and will likely be just fine, at least till the fine tuning stages. |
| Author: | BarnaciousC (registered user: 8 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 17:05 ( . ) |
another option I was considering would be to use a tone control such as used in the ac10 and the ac15 (which was based on this amp)where a pot and cap are placed after the PI like a cut control. this only leaves a volume control, would placing a 1M pot before the PI cause issues with the biasing as with the tone stack? |
| Author: | BarnaciousC (registered user: 8 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 17:08 ( . ) |
plus I am leaning toward restoring the amp and maybe building the mullard preamp that the amp was originally designed to run with. |
| Author: | marsvolta (registered user: 92 posts ) |
| Date: | Sun, Mar 18th, 2012 @ 20:51 ( . ) |
Sounds like a good plan. HiFi tube amps are the hot item these days. Keep it and take care of it. ;) |
| Author: | jazbo8 (registered user: 455 posts ) |
| Date: | Mon, Mar 19th, 2012 @ 04:15 ( . ) |
Restore the amp! These old amps are worth preserving, I think you are on the right track with the matching preamp. [link] Jaz |
| Author: | BarnaciousC (registered user: 8 posts ) |
| Date: | Mon, Mar 19th, 2012 @ 04:31 ( . ) |
thanks for all your help and advice, I'll keep you posted on how it goes. |
| Author: | mhelyar86 (registered user: 1025 posts ) |
| Date: | Mon, Mar 19th, 2012 @ 09:56 ( . ) |
I don't know a lot about vintage valve HiFi amps, but that's an amp I've heard of. (I'm assuming it's vintage, I can't view the pics from here). If it were mine and I didn't want to keep it, I'd research what I could sell it for in its current condition vs fully restored and try to make some money from it to spend on a fresh amp. Sounds like that's what you're doing anyway, so best of luck! |