Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frankeg on March 31, 2013, 10:14:04 am
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Happy Easter all,
Just so you know, I've read most of the previous posts on this subject and now I just want to be certain I understand it and can use it. I built an up-graded PR clone with Allen TP25 PT (310-0-310 @150 ma) and Allen TO20 OT (6.6k primary), the stokes PI mod and a bigger cab with a 12" speaker which sounds wonderful but sometimes lacks the clean volume/headroom I need for bigger venues. Many of those places have poor sound systems so miking the amp usually isn't a good option. I'd like to change the output tubes to 6L6's and from what I've read, all I really need to do is re-bias, based on the changes I applied to the original build. Is this correct or did I miss something?
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Due to the PT and OT that you have presently installed, I believe you're correct that only a rebias will be needed.
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I built an up-graded PR clone with Allen TP25 PT (310-0-310 @150 ma) and Allen TO20 OT (6.6k primary),
I think the TO26 would probably be better for 6L6s. http://allenamps.com/parts.php (http://allenamps.com/parts.php) The TO20 looks like its made for 6V6s or EL84s and might not have sufficient power rating to handle 6L6s (but only going by what they say on the Allen website for this).
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upgrade iron to vibrolux/vibroverb iron. PT is 325-0-320 @ 210mA and OT is 4K @ 35W. perhaps what you really are needing is a single channel vibroveb based on the type 6G16 ckt?
respectfully,
--pete
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Thanks for the reply's. I agree that the bigger iron is the ultimate way to achieve my goal so I think I'm going to build a head using the Princeton chassis and bigger iron. I am going to install the 6L6's in my current amp just to hear the result. I realize with my current iron, I won't be getting much more power. I wish I would have saved the site but I remember coming across a head build like this from johnny crash? I believe he also had a reverb pan in that project. Thanks again...fg
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I have a Classictone PT, 330-0-330 at 120 ma. I installed a gz34 and have 495 vdc on the plates and am using 5881 bias at 42ma. Mercury OT at 6k6 multitap 8,4,2. It is a princeton AA1164 circuit and everything is running smoothly. I tried the stokes and the Paul C, the removed them. Made too much floor noise for my taste. Not a lot, just more than I prefer. Instead of moving the PI up one node I decided to split the difference in voltage. I simply increased the voltage by lowering the last dropping resistor which put a little more on the PI and reduced the plate load resistors to 91k on v1 and changed the 68K on input 2 to 33k. Added a marshall mid pot. The amp sounds great. I did a few other things like changing bypass cap values, but that was to remove the mud. I put it into combo with 2, 10's. One Celestion Gold and the other a Celestion Vintage 30. Word is the Paul C mod gives a more symmetrical distortion, but the tradeoff is slightly harsh highs like a Pro Reverb. I am sure it could be worked out, but I liked it best without it.
The amp will run 5881, 6v6 and 7c5. The tremolo works great with the USA Tung-sol 5881 and I also have a switchable ss rectifier in it which pushes it to 512 vdc on the power tubes. The amp is beautifully clean and begins breakup at 9 o'clock and moves to singing distortion when dimed.
Been using it almost daily and the PT is not even what I would consider warm. Humbuckers and single coils both sound great with the single coils having a snappy response not common with Princetons. It is my no 1 gigging amp. It is a great circuit to modify, but in its stock form I found it to be to bass heavy and caused the speakers to flap.
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Ed,
Thanks so much for all the useful info! What you did pretty much sums up what I want to do. I'll probably stick with the 6L6GC's for a bit more clean but otherwise it's the same idea. I've read elsewhere in this forum about changing the power rail resistor's and that made good sense but I'm glad you actually tried both and shared your results. I used an upgraded tone stack (TMB) with my original PR clone which I liked and plan to do so with this build as you did. Did you use a PR chassis? That is my final concern - that the bigger iron won't fit or will be too heavy... Thanks again for sharing. frank
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I made the chassis from a Mojo drawing (attached), but it is like a princeton except I made the holes closer together to allow for more knobs on the front. I have a Volume, treble, Middle, Bass, Reverb, Dwell, Speed, Intensity and Depth. I also put a loop on the front as well. I prefer loops on the front if they will fit and not cause lead dress problems. I did make it dual bias by using a dual intensity pot. I attached a few other things.
I have a sheet metal break and a tig welder and a full set of punches. The chassis is custom, but Weber has one with the holes close together leaving more room on the right if you want to drill additional holes. The one they have you could easily add 5 holes to the face if you wanted.
The 5881's are not really going to limit headroom and need a little less current. It is up to you, but I have had 6l6's in and there was really no difference. I just happen to have more 5881 NOS and that is why I am running them. Let me know if you need any other info, the amp turned out really nice and I got a lot of help.
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Here are a few more attachments which may help. I did not use the master volume. The layout shows a crossline MV with a switchable pot. Tried it and did not like it. If I were only doing a single bias I would use a PPMIV master. Sluckey told me it may put too much on the bias section to use the LarMar PPMIV master. I did try it and it sounded really good, but I did not want any problems so I removed it.
I also changed the first 2 cathode bypass caps. The first one is 4.7uf/1k5 and the second is 3.3uf/820ohm. I simply tweaked it by turning up the bass all the way and reduced values until there was no mud using humbuckers. These values with my speakers made it so I could dime everything and get no speaker flap. Lowering the plate voltage on v1 seemed to remove all the harshness in the upper mids and highs, but I am sure you know what tone you are looking for.
Like I said, it turned out very nice with the ability to have mid scooped Blackface but bring up the mids and add a little boost and it will crunch much more than I expected.
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WOW! I may have to name the amp after you Ed! That is a lot of useful information! I sincerely appreciate it. How does the effects loop work? Meaning does it sound good? The theory says it can work but most makers use buffered effects. You're the 1st I seen use it. The standard PR chassis from mojo should work for me. I'll use the vibrato holes (speed & intensity) for the extras. I'm only adding the mid pot and perhaps the master vol. Thanks again. fg
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The loop works great for modulation effects. The schematic is wrong in that the connection to the jacks. Willabe gave me these I have attached. I only use loops for Modualtion and I always have a booster/EQ on front of them. That way I have volume control and tone control over the effects. Really, what I wanted to do was be able to have dealy and reverb without running the delay through everything, but it worked so well I am using all modualtion through it and using overdrive and compression in front.
I wouldn't name it after me. If you were to name it after everyone who had a hand in it it would be called BarrySteveBradPeteJohn, well you get the idea.
These schematics have some additional options you may like. I had the amp built before I installed the loop, it was not preplanned, so I used a combination of two of them. I did reduce the resistor 1 meg and added a 1 meg pot, but decided it worked well without it, so I used the pot for a depth control on the tremolo.