Carmen Ghia clone value questions

[creekrat]

So i scrapped my matchless ao-43 clone and have decided to go with a Dr Z Carmen Ghia/Hammond ao-35 clone.  I am using the Phoenix schematic and layout diagram but have a question about a few values.  Mainly the 500/10 watt and the 130/10 watt cement resistors.  Are those 500k and 300k respectively?  Also, does my spreadsheet match up with what y'all are seeing?

[TitaniumValhalla]

So i scrapped my matchless ao-43 clone and have decided to go with a Dr Z Carmen Ghia/Hammond ao-35 clone.  I am using the Phoenix schematic and layout diagram but have a question about a few values.  Mainly the 500/10 watt and the 130/10 watt cement resistors.  Are those 500k and 300k respectively?  Also, does my spreadsheet match up with what y'all are seeing?

They look like 500 ohms, and 150 ohms in the schematic. Definitely not 500k or 300k, which would be way too high.

[PRR]

They have to pass a lot of current, with low voltage drop. Yes, 500 and 130 make sense.

But "standard values" have changed over the years. 500 and 130 may be hard to find.

470 is close-enough to 500, and may be easier to find.

The 130 cathode bias is somewhat critical. Either 125 or 150 will work. Or two 270 Ohm 2 Watt in parallel. (10W was over-size.)

[kagliostro]

I don't know what is changed and why, but here there is a 1.4 version





Franco

[MFowler]

I've built a few of these both in the Hammond organ chassis and new chassis build.


Here is a BOM which you may already have.  Also the Botany Bay schematic is correct.

[kagliostro]

I'm not sure, but I think the 1.4 version is relative to this schematic (there is a CUT control)





Franco

[OwnGoal]

I know this is an old thread, but I built one of these clones a year ago, and I've never seen the schematic with the cut control. What does it do?
Is it worth adding to my amp?

[kagliostro]

See to the VOX AC30



there the cut control is present to ....... tame high frequency (or a Treble control if you want)

Franco

[pdf64]

If expanding to 2 knobs of tone control, consider trying Merlin's Bone-Ray design, as it will provide more flexibility whilst still having the low insertion loss (and similar action to) DrZ's Carmen Ghia Mids control, eg https://www.tristancollins.me/guitar/merlin-high-gain-preamp-firefly-output/

[vampwizzard]

I know this is an old thread, but I built one of these clones a year ago, and I've never seen the schematic with the cut control. What does it do?

youre blending phase inverted signal so that some of the signal is interfered with. Master volume blocks it on all signal, adding the cap makes it a filter and starts cutting high frequency.

Is it worth adding to my amp?

It depends. If you find the AC style circuit too bright and you cant control the ice pickiness it can be really helpful. There are other ways people have tackled this problem.

[porkzo]

Hey I know this is an old thread but I'm currently in the final stages of building an AO-35 based amp following the schematics and diagrams provided in this thread. I noticed that there seems to be a discrepancy between the diagram and schematic in regards to the grounding of the power filtering capacitor can. In the schematic, if I'm not mistaken, the preamp grounds are tied to the chassis: where-as the capacitor can is connected to the power star ground. Am I wrong in concluding that, in the diagram provided, the can seems to be connected to chassis ground with the Mains earth ground? Furthermore, is the separation of grounds in the schematic to minimize noise? Would tying the two grounds together effectively remove the earth by tying the Mains neutral and earth together? Any input is much appreciated!

Cheers

[PRR]

Welcome.

> tying the Mains neutral and earth together?

No, "Mains neutral" should NEVER be tied to ground/earth. Where do you see that?

[Willabe]

Grounding;

http://www.valvewizard.co.uk/Grounding.pdf

[acheld]

This can be a confusing topic to someone new to circuits, especially those we see in our valve amps.  And while Merlin is an established expert, and I generally agree with his advice, I think the chapter you are referencing is difficult to understand for many readers (including me).

Am I wrong in concluding that, in the diagram provided, the can seems to be connected to chassis ground with the Mains earth ground?

No, you are not wrong.   The key to understanding this is in the first page or two of Merlin's grounding chapter, when he admonishes us to think about current, which does NOT normally flow from the chassis ground to the Mains Earth Ground. 

Furthermore, is the separation of grounds in the schematic to minimize noise?

Yes.

Would tying the two grounds together effectively remove the earth by tying the Mains neutral and earth together?

Well, no.  Recall that Mains neutral (this is the white wire in the USA) is tied to one of the primary windings of the PT, and is NOT tied to ground.

It is true that chassis plays a role in most guitar valve amps circuits by functioning as a "return" for the secondary windings, but it is NOT functioning in this manner for the primary windings (to which it should not be tied).   

There is NO single grounding system that works every time.   However, if you pay attention to Doug's grounding scheme as outlined in his build documents, it is very reliable.  In general, ground your power amp section close to the transformer, and ground your pre-amp as far away from that as you can, generally close to your grounded input jacks in a Fender style amp. 

Best of luck, and have faith you will enjoy the journey!




 

[porkzo]

Thanks for the quick replies. I decided to tie the capacitor cap to star ground like it is in the schematic rather than to the chassis as in the layout. When you refer to Doug is this Uncle Doug or another? Where/what are the building documents you are referencing?

Cheers

[Willabe]

When you refer to Doug is this Uncle Doug or another? Where/what are the building documents you are referencing?

He's referring to Doug Hoffman, the owner of this web site.

Here's a link to the docs, it can be found at the very bottom of any page in the forum;

https://el34world.com/schematics.htm

[porkzo]

Thanks again!

[porkzo]

I've been trying to get this thing running for the better part of three days now. I've traced over the board and schematic, checked all ground connections and for cold solders/shorts; all multiple times. In the process I did manage to find a few forgotten connections which have since been fixed. I used a lightbulb limiter and variac for extra precaution with a dummy load or 8 ohm speaker always on the output. I also have a scope on standby. A few things that I can't manage to figure out.

1. With only the 5Y3 and preamp tubes plugged in the cathode Voltage on V1a/b is sitting at 150V and the Plate Voltage is sitting at 300V (Variac @ 50%). According to the notes in the Botany Bay Schematic the values should be 1.40ish V & 160ish V respectively

2. Absolutely no signal makes it through to the plate of V1a thus, the rest of the amp

3. With all tubes in a loud 60hz buzz creeps up as the tubes warm - this oddly disappears when the power tubes are switched from their sockets. ie same tubes but switched

4. With a limited probe with the scope the tone knob seemed to amplify a sine wave (maybe 60hz) when turned up. (dummy load was connected during this test)

5.Power tube Cathodes sit at 800mV (Variac 50%) -it should be 10.4V according to schematic.


I was trying to re-use as much of the original components from the AO-35 as possible but at this point the only original remaining parts are: R32, C18,  C6, C7, R13-15, R17, C14, the sockets, the OT & PT.


If any of this seems to point to something specific I'd be grateful for the insight. At this point I have ordered some more EL84s on the suspicion that faulty tubes may be to blame for part of the anomalies at play.

Cheers

[sluckey]

About your grounds... All the grounds must connect to chassis. However, the preamp grounds will connect to chassis somewhere near the input jack. And the OT/power grounds must connect to chassis near the power transformer. You should read zero resistance between the preamp grounds and the OT/power grounds. This resistance is being read through the chassis itself.

Your voltage readings will only be valid if the amp is connected directly to the wall convenience outlet. You can use your variac if you set it to 120VAC output, but... DO NOT USE THE LIGHT BULB LIMITER (LBL) WHEN MEASURING VOLTAGES! If the LBL did not glow very brightly when you had all tubes in the amp, then it's job is done. Put it under the workbench.

1. With only the 5Y3 and preamp tubes plugged in the cathode Voltage on V1a/b is sitting at 150V and the Plate Voltage is sitting at 300V (Variac @ 50%). According to the notes in the Botany Bay Schematic the values should be 1.40ish V & 160ish V respectively

This sounds like V1 cathodes are not connected to ground. This would be the case if you have the OT/power grounds isolated from chassis. You must have zero ohms resistance between the bottom side of V1 cathode resistors (re. schematic) and the PT center tap. If not, find out why.

5.Power tube Cathodes sit at 800mV (Variac 50%) -it should be 10.4V according to schematic.

Either set the variac for 120VAC output or plug the amp directly into the wall. When set to 50%, the variac  voltage may be so low that the tube heaters ane barely (if at all) heating.

Once again... PLUG THE AMP DIRECTLY INTO THE WALL TO MEASURE VOLTAGES! Otherwise, your voltages will never match your schematic.

[porkzo]

Just wanted to say it was 100% that Signal and Power ground were not connected. I now have a working amp, albeit a little bit of loud hum when above 1 o’clock but still  very satisfying to complete this build; thank you for getting me passed the finish line.

I’m wondering if anyone knows if it’s normal for the Carmen Ghia tonestack to be pretty bassy below 1 o’clock? I think I might have it wired “backwards” as it goes from bassy-trebly clockwise but it feels like most of the tone sweep is relatively unusable.

Also does anyone know if there is a way to have a multiple output impedance configuration Setup with this build without replacing the OT with equivalent centre tap OT or a Weber Z matcher? I’m hoping to set up a cab with two 8ohm speakers that I can use as single speakers or dual in either a 4 or 16ohm configuration. If this isn’t really possible with maybe an external transformer or inductor I’ll just bite the bullet and set up the cab to be 8ohms after both speakers are wired but thought it would be worth checking in case somebody knows.

Cheers

[mresistor]

I built my CG using a Hammon AO-35 chassis.  The tone knob on mine when max ccw is very trebly and when max cw is bassy with way less treble.  It's working backwards from yours I guess. It doesn't bolther me at all and the amp is a real keeper. Can go from glassy cleans to narly easily with lots of tones inbetween.  I mounted mine in a self-made little marshall-like head cabinet like a jcm with salt and pepper type cloth on the grill. Mine is 8 ohm out only IIRC which works for me. I like the stock OT.

[porkzo]

Hmmm, I'm finding mine to be 80% bassy and finding it hard to find a clear tone even on the bridge pickup of my SSS Strat. If I wanted to adjust the tone sweep to be less bassy would I be raising the value of the 250pf cap or the 10nf cap? I'm leaning towards it being the 250pf but I'll admit I'm not quite sure how the tone circuit works. I'll note I'm running it through the stock 8 ohm Eminence speaker found in a Hot Rod Deluxe, not sure in what direction this would shape the tone.

Cheers