Powered up my custom build and...............

[hesamadman]

Ones the power and the other is standby

[hesamadman]

I wonder if you're about to tell me to check my ratings on the switch. I didn't think of that. Oops.

[Willabe]

I'm not sure those switches will be able to handle the power, voltage and current.

Be safer to use a larger body switch with higher voltage/current handling.


             Brad   

[Willabe]

I wonder if you're about to tell me to check my ratings on the switch. I didn't think of that. Oops.

Yep.   

[hesamadman]

I'm really glad you caught me on that. No it won't handle it. Not even close actually. Ha. I'll have to swap that thing out.

35vdc 

[sluckey]

Do you really need a standby switch on a breadboard?

[Willabe]

A Carling SPST switch like Fender used for decades, for on/off and standby switch, is rated at 6A/125v, 3A/250v.

They get away with using a switch under rated for voltage because their not drawing, more than what, 400mA's at full tilt in a 4x6L6GC 80w Twin?

Doug's got them and other switch's that will work.


                  Brad   

[hesamadman]

Thanks Brad. I'll check into those.

I guess not Sluckey. The standby on this build is optional anyway.

[Willabe]

Thanks Brad. I'll check into those.

Here's a link to Doug's store, switches section. 

http://hoffmanamps.com/MyStore/catalog/parts7.htm

And you can find it at the very bottom of the page.


                  Brad   

[hesamadman]

Got her all wired. Power amp and one full triode preamp (with pre volume in between stages. I want to add a PPIMV so I can get an idea what this preamp will be like without having to have it full blast. Can you check this and tell me if its ok. I dont think I have to add anything since its a cathode bias but I may be wrong 

[sluckey]

I would put the cut control before the MV.

[hesamadman]

will do. Will report back tomorrow 

[hesamadman]

After a lot more patience and way more hours of work than I should have spent in a few days, the amp is up and working. There are a few things that I noticed that I would like to tweak. The preamp volume knob is too drastic. It's a 1m, which is what's on the 2204 but I've been told 500k is standard.

The tone stack doesn't seem to affect the frequencies as much as I think it should. The bass seems to. But the mid and treb knobs have little affect on the tone. It is a 2204 tone stack.

It is also a tad gainy and muddy. I know the preamp values need to be adjusted. I assumed the muddiness would be a result of cathode cap value being too high but the only cathode cap I have is a 4.7uf. I did goof and placed it on the second half of the first triode rather than the first. So the first half of triode only has a 1.5k. Not sure what affect this has but I will play with that.

[hesamadman]

Been playing around with the amp over the weekend. I got all the bugs worked out except one. I was hoping someone could help me out.


I installed a PPIMV. It seems to function fine except when the amp is comepletely cranked (all the way up and maybe just a tad shy from being all the way up) I get some wild oscillation like a consistent thum-thum-thum-thum. Im using a dual 500k. I also installed the cut control before the PPIMV. Im guessing it has something to do with the PPIMV not being wired with shielded cable??

[Willabe]

Im guessing it has something to do with the PPIMV not being wired with shielded cable??

Could be, got some shielded cable left?


                 Brad   

[hesamadman]

I'm all out but I'm ordering some from Doug tomorrow. We will see how that goes. I'm guessing it will probably take care of it. I keep double checking my PPIMV but there's just not a lot to it especially on cathode bias.

[hesamadman]

Sluckey,
You and i discussed my OT a few times. i was just wondering if you can tell me what kind of differences I could expect if I were to change out to an OT designed for this circuit. Like I mentioned before, right now this is for a 30 watt marshall. I have what I feel is a bit of woofieness in the low end. It can be dialed out but the low end is pretty dominant even with the bass being set somewhat low. I really don't think its in my preamp design as I don't have any low pass filters and the only cathode bypass cap I have is a .68 on the first triode. I actually have multiple hi pass filters. Just curious what you thought.

[ampgeek]

Enjoyed scanning through this thread and seeing your approach!

I suspect that the high valued cathode bypass and coupling caps are contributing to the woofiness that you describe.

All of these changes will help to dial out the high bass content that I think you want to reduce.  These are based on the schematic presented early on in the post.  Please forgive me if I didn't catch any changes to these values that have been made and possibly overlooked at this end.

C1a to 0.1 uf
C1b to 0.1 uf
C2 to 0.022 uf
C5 to 0.1 uf
C9 to 0.022 uf

If that gets too shrill, increase them halfway between the suggested and current values.  Or...start by changing to values between what you are at and the suggested reduced values.  Try doing them one at a time so that you can calibrate your ear to the difference that each one makes.

Rock on,
Dave O.

[ampgeek]

ooops....forgot to also mention C14 (power tube cathode bypass cap).  I haven't built many cathode biased output tube amps but will mention that the same philosophy applies there as well.  Lower values will decrease the low frequency content of the signal.  I am not sure if halving C14 is advisable due to biasing needs...but....I am sure that reducing it by 25% will make a noticeable difference.

Hopefully someone else with more experience will confirm that feeling.

Good luck!
Dave O.

[hesamadman]

Thanks a lot for the response Dave. Actually thats my fault. I should have menioned that the preamp schematic posted earlier no longer applies. I have since changed all coupling caps to .022 and I only have one cathode bypass cap in the preamp. The first triode consists of a .68uf bypass cap. I am in process of redrawing schematic. If I dont get much activity on here I will post it in a day or so.

[ampgeek]

No sweat mate!  It is nearly impossible to capture in print every little change made to a prototype amp..  Trust me....I know...and I seem to keep making the same "mistakes" over and over again.  Fortunately....sometimes they are happy "mistakes"!!  LOL

I saw mention of the 2204 preamp earlier in the thread.  Is that where you are at right now?

Cheers,
Dave O.

 

[hesamadman]

No sweat mate!  It is nearly impossible to capture in print every little change made to a prototype amp..  Trust me....I know...and I seem to keep making the same "mistakes" over and over again.  Fortunately....sometimes they are happy "mistakes"!!  LOL

I saw mention of the 2204 preamp earlier in the thread.  Is that where you are at right now?

Cheers,
Dave O.

The tonestack setup is at least. HA. The first preamp tube is probably close with that .68uf cathode cap. But all other parts of preamp were tuned to taste. I had to do a lot of gain reduction since the original preamp design. I pretty much have 100k plate resistors on the entire preamp and I think the smallest cathode resistor I have is a 1.5k. Also have a 10k on cathode resistor and the last cathode i believe is about a 4.7k. I have a standard high pass filter on preamp volume. And another on V2a. Also opened up a couple grid resistors a tad.