AC30-type Circuit with Channel Switched OD?

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lord preset
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AC30-type Circuit with Channel Switched OD?

Post by lord preset »

A few years ago I built Hoffman-style AC30 clone for a friend with a toroidal PT and a 40 watt Fender HRD OT. Details here: http://ampgarage.com/forum/viewtopic.ph ... cheap+ac30 . I am now pondering making one for myself (TB channel) in a Princeton-sized chassis and a smaller cabinet. I am also pondering the possibility of adding a switched OD channel, but I am not aware of any real examples to go on. I don't mean just any EL84 circuit with switchable OD (i.e. Peavey Classic 30 and Laney others -all of which sound like rat farts IMO), I mean something pretty close to an AC30 circuit with an added OD channel. Is there a reason why I'm not finding examples - i.e. does this not work out well given the AC30's preamp/tonestack design?
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Re: AC30-type Circuit with Channel Switched OD?

Post by pompeiisneaks »

I've been designing an amp with two channels, the EF86 normal channel and a top boost channel with a dumble style OD that's switchable, but not completed it yet. From what I understand, though some people say the OD into the EL84's doesn't sound right, so I'm not yet sure how that part will play out. I'm open to feedback too, on what would, and wouldn't work.

~Phil
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lord preset
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Re: AC30-type Circuit with Channel Switched OD?

Post by lord preset »

pompeiisneaks wrote: Wed Nov 08, 2017 4:28 am I've been designing an amp with two channels, the EF86 normal channel and a top boost channel with a dumble style OD that's switchable, but not completed it yet. From what I understand, though some people say the OD into the EL84's doesn't sound right, so I'm not yet sure how that part will play out. I'm open to feedback too, on what would, and wouldn't work.

~Phil
I would be interested to see how it turns out. I have a Valvetech Hayseed AC30 clone that I like a lot but it, like other AC30s is too heavy for grab and go use. It has an EF86 normal channel that IMO is not that useful by itself - kind of dull sounding - but it sounds great driven hard with a treble booster. I have not had good luck with EF86s going microphonic in smaller combos so I want to stick with the Top Boost channel. I've hear the same thing about EL84s and OD but on the other hand the AC30 circuit tends to play very well with OD pedals so I don't quite know what to make of it.
10thTx
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Re: AC30-type Circuit with Channel Switched OD?

Post by 10thTx »

I'll offer some thoughts for consideration. I did a D'Mars ODS build that used sort of a Marshall preamp topology into a Dumblish inspired overdrive, LTPI and power amp that was cathode biased. I've built two of those and they turned out great, IMO. One used 5881's and the other 6V6's.

I simply mention that to affirm that one can do something outside the box and have it turn out well in adding overdrive.

Having said that, DaGeezer (who used to be a moderator on the Hoffman Forum & participated on TAG also) took several Dumble inspired preamp and overdrive designs and ran them into a power amp using EL84's (using passive FX loops on both amps). He shared the sound clips with me from that experiment where the EL84's were being overdriven by the OD channel. I did not care for the tone. It sounded somewhat too buzzy and distorted to me. (I don't have a copy of those EL84 sound clips) He then did the same experiment with a quad of 6AQ5's and it sounded fantastic (which was the norm on his amp builds). I will own though that I have never bonded with an EL84 amp.

IF I were attempting this experiment, I would be inclined to have a pair of EL84's cathode biased along with a pair of JJ 6V6's that are cathode biased.

Last thing I'll say is that I built one amp where the OD channel overdrived the amp too much into distortion, so I did an OD channel with a triode and cathode follower and got a really nice tone out of it. The other option might be to try a 5879 pentode as the OD channel. I've done that before and liked the results. The advantage of the 12A_7 approach is you can choose the tube you want for the amount of gain such as a 5751 for 70% gain.

I've attached a SCH editable schematic if you want one.

With respect, 10thtx
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10thTx
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Re: AC30-type Circuit with Channel Switched OD?

Post by 10thTx »

5879 OD channel.
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10thTx
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Re: AC30-type Circuit with Channel Switched OD?

Post by 10thTx »

I thought of one other possible approach using a 5879 V1 for a fat chimey tone. I've used the 5879 in V1 on several HoSo56 builds and really like it.

This would simply add a Hoffman style "hot switch" triode in the mix for a boost effect. You could put this on a relay.

With respect, 10thtx
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