Custom 50 Hiwatt project...

[TubeGeek]

Hey guys,

I finished building my custom 50 hiwatt head and holy smokes this thing is cool.  It certainly adds a nice tone to my collection.  I love the hallowed clean tone and I also like how it breaks up when pushed LOUD.   I will be experimenting with different speakers and cabinets over the next few days or weeks. A few months ago I was able to purchase two original Fane speakers which will hopefully be a match made in tone heaven.  I'm gonna wire them up and try them tomorrow.

This amp powered up without any issues and is extremely quiet in operation...I had to double check it was even on and connected to speakers...turns out there is an issue... See below.

A little story of this amp...

I had several hiwatt sized chassis made for me a while back and have been using them on my personal projects.  I built an ac30 (hoffman style) with one of the other chassis.  A while back I had a customer ask me to finish building his vintage hiwatt restoration custom 50 kit...I decided at that time I would build myself one as well.  The reason this guy wanted a hiwatt is because he showed up to my shop when I lived in UT while I was testing a vintage hiwatt after servicing and he loved the tone I was getting.  So did I.

This amp was built from scratch.  I followed the dr504 layout on Mark Huss' hiwatt website only mine had a few differences.  When I worked on that old Hiwatt in Utah I was asked to modify the input so that the normal and brilliant channel vol pots were blended together.  I looked it up online and it was a popular mod to this amp because the triodes of v1 are in phase and thus the mod works well, no phase cancellation happens when the signal is sent to both v1a and v1b equally.  I really liked this function as it allowed a nice blend of the beefier normal along with just the right amount of brilliant shrill.  This is why I have only one input wired up.  I can add another input if I want, the hole is plugged for now.

There were a few more changes to my amp...I built the late 70's two input version.  There are a few subtle differences in gain structure and in the PI but they are close to each other.  I may experiment with those configurations at some point when I learn the sound of this amp more.

I have telefunken's in v1, v2, v3 and an rca 5751 in the v4 position.  I have a matched set of Winged C el34's in the power section.

I began trying to follow the insanely nice Harry Joyce layout but gradually incorporated my own ideas.  I didn't want to run a bunch of jumper's under the turret board because I may want to modify or tweak things a little and that is just a pain in the ass to get to if I want to change things so I ran the jumpers on the top of the turret board.  I used teflon wire throughout this amp.  I did upgrade the bias circuit from the original circuit too, the original was fixed and came in at around 80mA per tube so I installed the adjustable circuit and adjusted for 40mA each tube.  The bias pot is held down in the chassis very well with a cool 3M two sided tape that sets like super glue after a day or so.  It is secure.

I'll let the pictures do the rest of the talking.
 

[kagliostro]

Fine 

if you can we would like to see link to the schematics of the mods you put on

Kagliostro

[gldtp99]

Nice job---- i've built several Hiwatt circuit amps--- some i would call clones and others are more Hiwatt circuit-based and not really a clone of any Hiwatt model.
I've kept one DR504 clone for myself---it's a Mid 70's 4-Input circuit----housed in a white Swanson headcab---- it's third from the top in this pic of white amps i have:

[LooseChange]

Really nice work!!

[TubeGeek]

Fine 

if you can we would like to see link to the schematics of the mods you put on

Kagliostro

I didn't have schematics of the mods.  What I did was follow the early 70's four input layout from mhuss' website and made changes according to the late 70's two input schematic.

The only real mod so far was the input goes to both grids of v1 to get the "blend" between channels.

[TubeGeek]

Nice job---- i've built several Hiwatt circuit amps--- some i would call clones and others are more Hiwatt circuit-based and not really a clone of any Hiwatt model.
I've kept one DR504 clone for myself---it's a Mid 70's 4-Input circuit----housed in a white Swanson headcab---- it's third from the top in this pic of white amps i have:

Nice amps!

Quick question...do you use a 1A fuse for the B+? 

I am blowing the B+ fuse when I crank up the normal and brilliant vol pots and lower the master vol.

[gldtp99]

Yes, i'm using a 1A slo-blo for the HT fuse----i've never had this problem---i'd first suspect a bad preamp tube--- or maybe one of the preamp filter caps isn't working properly when preamp is pushed.
I used JJ filter cans in mine and i have all RCA 12ax7a's in preamp----i like a 12ax7 over a 12at7 in the PI in this particular amp.
Normally i'd first suspect a bad output tube but you say it blows the HT fuse only when preamp is cranked and MV is turned down---does it blow the fuse when preamp is cranked and MV is set high ? This will be a loud test !!!!..................gldtp99

[TubeGeek]

I haven't done the loud test yet.  Jeez these are loud amps eh!

I used new jj caps in my build too.  

The tubes test out good on my hickok tester and in other amps.

I spoke with Mark Huss today and he thinks i may have an oscillation in the preamp that i can't hear.  He recommends looking at the output signal on an oscope to see if there is a high frequency oscillation as i turn up the preamps.  This would cause too much current flowing in the power tubes and thus the blown b+ fuse.

To be honest i have not really been using my oscope to troubleshoot amps and i am a little unsure of what to do.  I know it seems obvious but can i poke around in an amp without worrying of probing something that is going to blow up the scope?  I would think this is ok as long as the probe is set to 10x correct?

Next question... Using the scope where would i monitor the output?  At the speaker jack? On the grids of the pi?  On the grids of the output tubes? On the plates?

[HotBluePlates]

Do not monitor the signal on the output tube plates, as many scopes will not handle the voltages present. Monitor at the output tube grids and/or at the speaker output.

If you have a dual channel oscope, you can monitor the test signal on one channel, and the speaker output on the other, and adjust the vertical sensitivity (and phase, if necessary) to see each at the same time. On some scopes, you can make sure they are out of phase and add the channels, which will show you any distortion and oscillation. However, it is probably more helpful to have at least some reference input signal to compare against, because distortion would be harmonically related, where oscillation would very often not (and oscillation will generally be very much higher than any normal harmonics).

[Fresh_Start]

Dayum!  Even your chassis cradle looks great...

Cheers,

Chip

[TubeGeek]

I connected an o-scope to the output jack and inserted a sine wave of 400Hz into the input.  I also am monitoring the bias current while I troubleshoot.

The first thing I notice is it does not take much turn on either normal or brilliant channels before the onset of distortion.  I'd say by the time I turn either vol's to 3 I can see a square wave on the scope.  If I continue turning either of the vol pots up I see the sine wave turn to sqaure and the when the vol's are maxed I see another hump on the bottom portion of the waverform.  See pics I snapped on my phone.

It is clear to me that there is a high current draw somewhere because I see the bias current go very high from 40mA when the vol pots are down to 0 and then the bias currents jumps very high peaking at 230mA when either preamp vol's are around 7 or 8 on the pots.  It performs this way with either normal or brilliant channels on their own or together.  I can also see red plating on the el34's, which verifies high current draw.

The amp sounds great but I must have something wrong in the preamp to cause the excessive current and blowing the b+ fuse continually.

pic 1 is with the preamp vol's around 1 and master vol on 2.

pic 2 is with the normal vol at about4 or 5 and master vol at 2.

pic 3 is with the normal channel vol pot at max and master vol still at 2.

Troubleshooting with an o-scope is new to me and I am trying to process what is going on here but I could sure use any advice and help on taming this beast.  The best part is that I know I am going to learn some stuff through this process.  Not to mention that Hiwatt's are not my familiarity either.

I will go over the schematics and layouts I followed again making sure I didn't make a mistake somewhere.  But I am hoping someone smarter than me will have an idea where to look in the circuit.

[sluckey]

I connected an o-scope to the output jack and inserted a sine wave of 400Hz into the input.

What is the voltage level of the injected sine wave? 100mV to 200mV peak to peak is all you usually need to connect to the input jack.

[TubeGeek]

The signal generator is at minimum level.

I have an idea of what the problem could be...it could be the way i wired the "blend" between normal and brilliant channels to v1.  Right now i have one shielded wire connected to the tip of the input jack going to pin 7 and a small jumper connects pin 7 to pin 2 of v1.  Maybe i should have two separate wires from the tip of the input jack going to each grid input.  Could it be this simple?  I won't know until i get home from work and rewire it tonight.

[sluckey]

The signal generator is at minimum level.

Yeah, but what is that level. It's important to know. My sig gen puts out zero volts at minimum level. You may need an attenuator to knock your level down to an acceptable input level. I like to connect the sig gen directly to one scope channel and set the trigger source to that channel. I use a "Tee" connector to also connect the input cable to the amp under test. Then use the second scope channel to look at the amp. By triggering the scope on the sig gen signal you'll always have a stable display without constantly readjusting trigger levels and you will see all the phase inversions or shifts thru the amp.

Maybe i should have two separate wires from the tip of the input jack going to each grid input.  Could it be this simple?

I don't think that will make any difference. A schematic would be very useful.

[TubeGeek]

I will check the level of the signal generator and post back.  I will setup the scope and signal gen as you say, makes a lot of sense.

I have attached the schematics for reference.  I used the late 70's two input version with the 50 watt power and output.  The dr504 full version sort of puts it all together in one, keeping in mind there are a few differences between the schematics but nothing too major that will change operation.

I will measure voltages throughout the amp tonight and post back.

[TubeGeek]

I spent my "amp time" tonight going over every connection, measuring voltages, comparing to schemtaics, layouts, etc.

I get between 20mV and 200mV on my signal generator.  It has a built in pad if I pull the knob out.  I didn't scope the amp tonight, I need to find a "T" bnc connector.  One thing I did notice tonight was...instead of connecting to a dummy load tonight I connected to a speaker cab while measuring voltages.  While monitoring bias current I did not see it move at all while raising or lowering the normal or brilliant channel vol pots.  Seems like I may have been overdriving the preamp last night when inserting the signal gen???

Here are the voltages I am measuring:

v1: 12ax7
1- 194
3- 1.87
6- 195
8- 1.88

v2: 12ax7
1- 158
3- 1.0
6- 158
8- 1.0

v3: 12ax7
1- 345
2- 62
3- 65
6- 247
8- 1.3

v4: (PI)
1- 334
2- 59
3- 65
6- 335
7- 56
8- 65

v5+6: el34
3- 476
4- 469
5- (-44)
6- 474

[Shrapnel]

I think a 1R resistor in the Cathode circuit would be good about now. Sometimes the right bias voltage doesn't actually provide the currents you'd expect. Do your EL34's behave similarly in other amps at the same bias voltage?

[TubeGeek]

Well I have been working on the amp these past few days and still have yet to find an issue.

Last night I installed another set of el34's and guess what?  The fuse does not pop with the new tubes.  Monitoring the bias current and everything is fine.

I reinstalled the winged c el34's...the fuse pops.

So...I guess I have been chasing a ghost.  It appears that there isn't a problem with the amp after all.  It must be a bad set of el34's.  They do test good on my hickok tester. Oh well.

I am able to turn both preamp vols up to max and the master vol without any problems.  

That's too bad that my winged c el34's won't work in this amp.  Now I am wondering why and if I should order another set or if I should order another brand.  Do you guys have a favorite el34?  

Now for some fun...actually playing the amp!  I'll put some mileage on the amp and hopefully no further troubleshooting is needed.

Thanks for the interest and help as always.

[TubeGeek]

I think a 1R resistor in the Cathode circuit would be good about now. Sometimes the right bias voltage doesn't actually provide the currents you'd expect. Do your EL34's behave similarly in other amps at the same bias voltage?

I have been monitoring the bias current with a bias meter which has the 1 ohm resistors...