JP Hiwatt

[TIMBO]

Hi guys, The build is comming along nicely, just a couple of question before i inject it with some volts.....

The large circle is the PRESENCE pot that i would like to remove (don't have room on the front pannel), can it just be disconnected from pin6 V3 and the 10k??

The small circle, is this part of the PRESENCE circuit, or the NFB, as i have not seen this in the normal FB circuit???

I was told that V3a sets the bias for the PI. I will have lower volts at node E than the original Hiwatt so will this upset the PI. Will it have to be adjusted the a particular voltage. A bit over my head with this one. Thanks

[HotBluePlates]

The large circle is the PRESENCE pot that i would like to remove (don't have room on the front pannel), can it just be disconnected from pin6 V3 and the 10k??

The small circle, is this part of the PRESENCE circuit, or the NFB, as i have not seen this in the normal FB circuit???

All of that is part of the Presence circuit, as well as the 10kΩ and 2.2kΩ resistors.

To avoid potentially changing the nature of the Hiwatt circuit, what if you used an internal trim pot for the 100kΩ pot? Or, set the value with a pot, then replace with a pair of resistors to provide a fixed setting. In my experience, most folks don't fiddle the Presence control a lot, except maybe onstage (if they're not used to the amp being onstage). A fixed setting, screwdriver changeable (externally available and mounted on the chassis) or internal trim pot maybe be the ticket.

I was told that V3a sets the bias for the PI. I will have lower volts at node E than the original Hiwatt so will this upset the PI. Will it have to be adjusted the a particular voltage.

Yes, to match the original amp's performance (generally to ensure the same range of available output signal voltage).

But what voltages are you using, given you're using 6V6's instead of EL34's? If all other voltages match (Mark Huss' schematic for the JP doesn't include voltages), change the value of the 33kΩ resistor to get the needed voltage.

If your other voltages are similar to, say, a Deluxe Reverb, make the cathode follower output voltage about a volt less than the cathode voltage indicated in that amp (75-76v).

[TIMBO]

Thanks HBP, The internal pot is a great idea as the presence is not something i do much with and as i would like to try and keep the amp as close to the original.

As for the volts required at node E i have wired it a bit different than the original only the make it easier to setout the circuit board . I was hoping that it would give the similar droping resistor as the 22k and the 10k but that 22k is in the Ceritone build layout and Mark has it as 47k  

I measured the node A (420v)without tubes in and E is about 400v but as you say the schem does not have any voltages to go by.

I'll will sort out the presence circuit and inject some volts and see what i get.Thanks

I just found that the voltage at HT1 (node A) is 460v if that helps. Thanks

[TIMBO]

Hi guys, All well with the start up (SQUEAL, switch the OT wires for a fix) and took voltages and the only concren is the voltage on the cathode of V4 being 51.6v and as HBP indicated could be around 75-76v so a possible adjustment to V3a may be needed ???? as the volts at V1 look to be in a good range.Thanks

Volt readings Sheet 3

[HotBluePlates]

... the only concren is the voltage on the cathode of V4 being 51.6v and as HBP indicated could be around 75-76v so a possible adjustment to V3a may be needed ????

That was a guess based on the voltages shown on one of Mark Huss' complete DR504 schematics, and the AA763 Deluxe Reverb schematic.

To drive EL34's with just over 500v on their plates, Mark's DR504 has 356 on the plate of one section of the long-tail. But you're driving 6V6's at lower plate voltage.

So let's assume you have sufficient drive voltage to your 6V6's; You show 260v on one section of your long-tail, which is 73% of the voltage shown on Mark's DR504. 73% of the 76v he shows on his amp's cathode follower output is 55.5v.

You could either increase the voltage to node E (by reducing the series dropping resistor), reduce the value of the 1MΩ going to the cathode follower grid, or increase the value of the 220kΩ resistor from grid to ground. Any of the 3 should increase the output voltage from the follower. Replacing the 220kΩ with a 500k pot wired as a variable resistor should help you dial in the exact value needed.

It may/may not make any obvious difference in the operation of the inverter. If the amp seems to function properly to you now, you may not need to tinker it.

[TIMBO]

Thanks HBP, I tried changing the dropping resistor (33k - 15k) and settled on the 22k as this gave a little higher voltage on V1 and V3. The sound wise, the changes didn't alter anything much but i do think that the volts that are there now sound the best.

Played with V3a and was able to get a close match to the AB763 Deluxe PI voltages by replacing the 220k resistor with a made up 352k (330k+22k). The V4 cathode now reads Pin8/3, 75.4 and Pin1 is 215v and Pin6 is 205v so this is good enough for me. 

I found the DR504OL schem that had the voltage readings that was of some help.
The amp is quiet but there is some fizzy/buzz at high volume that is reduced when touching the strings on the guitar as well when the footswitch is engaged, might need to investigate latter.
Even with some cheep peramp tubes in, it sounds damm good and the 6V6s are loud and a little chimmey but hit that power cord and it rattles your bones.     

The footswitch when engaged is a useable FX that i need to play around with when the amp is finished as the XLR socket is not in place as i do not have a faceplate yet.
I may mount the Presence pot on the back pannel, as i have three holes unused on the front that are a possible OD circuit. Thanks

[TIMBO]

Hi guy's, Finally got a chance to do some fiddling.

I rebuilt the PI as per Mark Huss's 6V6 plexi and found that i was unable to tell if there was any real difference to the original sound other than it had a little less bite.

I redid the PI to the original and tweaked the droping resistors as per my schem and i'm happy with the result.

HBP, i played around with the 220k (replaced with 352k) resistor to increase the bias volts on the PI cathode but have changed back to 220k as i found there was on change to anything

I found that i was getting a lot of feedback, i now think that it might be a case of the amp, guitar being too close to the speaker cab while tweaking.

My next problem is to tame the fizz/hiss when turning up the volume,so if anyone has any ideas

I have two chokes that i was going to try, the first is one that was in the amp that the PT came from, but all i can read on it is that it is :- 

ROLA
Type.....
14H at......
10 volts......
D.C. resistance...
550 Ohns.....
As the other half of the paper rapped around the core had disappeared

OR i use a standard fender type choke,not sure what Henrys they are, Will adding a choke help with taming the fizz/hiss. Thanks 

[TIMBO]

hi guys, My fizz problem is in V1. I tried afew ax7s but the fizz remained so i tried ay7 fizz was reduced so the next was au7 that was the best result .

I tried a few cap values across the 100k cath resistor (with ax7 in) and anything above 1uf reduced the fizz but make the sound a bit muddy.

Any thoughts how the fizz could reduced,Thanks 

[sluckey]

Replace the 220K plate resistor with a 100K metal film resistor. Might help. BTW, you don't want to put a cap across the cathode resistor in a cathode follower circuit.

[TIMBO]

Thanks Sluckey, I use 3w metal oxide on all plate res. I'll give a 100k a try. 

[TIMBO]

Hi guys, I been trying to tame the hiss/fizz for a while but don't seem to be getting any closer to getting rid of it 
I have found lots of different layouts that clearly show grounding, lead dress and component positions and still it remains 

Am i just trying to fix something that is just part of this circuit, as the CF follower is not a commom thing for a V1 position 

The input jack wireing is a bit confusing and if it could be wired as a normal HI/LO input would it achieve the same effect 

The overall sound at lower levels is great but with the footswitch ON the fizz is only at full volume and when OFF the effect is unuseable

So again i am faced with what to do. Thanks

[TIMBO]

Hi guys, With a bit more probing i was able to reduce the FIZZ a small amount by removing the bypass cap on V1b and V2b.

I also changed V1 as per the wizards prefered circuit this also helped a little.

The amp has a volume level that has an acceptable amount of fizz that is not too loud.

Unfortunately guitars that have humbucker pickups seem to produce more of the fizzy buzz and again is reduced when touching the strings. 

The strat is a different story its is the most quiet with very little fizz and even when the pickup switch is in position 2 and 4 

The input vol pot is also a little scratchy @ about 8 (out of 10) then the fizz increases to full volume   

[HotBluePlates]

By "fizz" then, you mean the buzz you hear when you don't have a good string ground? Like a buzz that stops when you touch the strings?

[TIMBO]

The fizz, maybe sounds more like white noise.

It is also present without guitar plugged in but only at full volume, i checked my tos and it is dead quiet at controls at 0 but it too has a small fizz/hash when controls are at full.

I do get some grounding buzz from my guitars as it seems to be a problem with the humbuckers but the strat is fine.

[HotBluePlates]

White noise may be unavoidable to an extent, when you have a lot of gain.

Could be a noisy tube, could be a noisy resistor (usually a large value like 1MΩ, but occasionally a cathode resistor) which then has a lot of amplification afterwards.

[TIMBO]

Thanks HBP, Its hard to know what these amps are like when there is not alot to compare too. So i don't know if it is just the Hi Gain circuit.

As i see it the preamp works fine, even the foot sw. but at low levels and to have 50-100w to amplify the preamp would be great but with the 6V6s i'm not getting that drive.

I used some of M Huss layouts to avoid these things but still i battle with a problem.Thanks

[HotBluePlates]

The fizz, maybe sounds more like white noise.

It is also present without guitar plugged in but only at full volume

I wonder if you could find the source of the noise by using a jumper wire with alligator clips on both ends. I'm thinking you could ground grids of various tubes stages to see where you can kill the noise, or not. Try to isolate the noise to a given stage.