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1  Muchless / DC 30 / Re: HF squeal around 12kHz in DC30 on: April 06, 2010, 11:51:41 AM
Contacted Nik and he suggested building a separate shield fot the OT. So I made a mock up and installed it and it removed the squeal.

Nik really is great with his help.

Cheers,

Crusty
2  Website, Store / Support / Re: HF sqeal around 12kHz in DC30 on: April 06, 2010, 11:50:23 AM
Contacted Nik and he suggested building a separate shield fot the OT. So I made a mock up and installed it and it removed the squeal.

Nik really is great with his help.

Cheers,

Crusty
3  Muchless / DC 30 / Re: HF squeal around 12kHz in DC30 on: April 06, 2010, 09:42:14 AM
Okay, checked tubes by swapping out.  No change.

Did some more trouble shooting and found that the squeal disappears when the cut control is maxed to knock out the higher frequencies.

Explored the wiring from the cut control and tone stack to the EL84s and noticed that moving the wires altered the frequency of the squeal.

Further investigation revealed that the squeal is definitely related to the current going IN to the output transformer.  Moving the wires for the output tranny makes all sorts of horrible squealing noises.

Is this a defective output tranny?

Cheers,

Crusty
4  Website, Store / Support / Re: HF sqeal around 12kHz in DC30 on: April 06, 2010, 09:25:18 AM
Okay, checked tubes by swapping out.  No change.

Did some more trouble shooting and found that the squeal disappears when the cut control is maxed to knock out the higher frequencies.

Explored the wiring from the cut control and tone stack to the EL84s and noticed that moving the wires altered the frequency of the squeal.

Further investigation revealed that the squeal is definitely related to the current going IN to the output transformer.  Moving the wires for the output tranny makes all sorts of horrible squealing noises.

Is this a defective output tranny?

Cheers,

Crusty
5  Muchless / DC 30 / Re: Looking forward to building a couple of these on: April 05, 2010, 10:26:17 AM
Hi Alwalt,

Well I finally got around to finishing one of the amps.  We'll be double checking and firing up the second one next weekend.  Then it's on to building the cabinets.

I have an issue with this first one (don't know if it's a characteristic of the amp until I finish the 2nd) but there is a high pitched squeal at around 12kHz that kicks in at around 30% volume on both the 12AX7 and EF86 channels when the master volume is >50% or bypassed.

Although it's hard to tell at the frequency, it sounds like a sine wave and the frequency shifts slightly when the volume level is changed.

Given the nature of this squeal appearing within the preamp stage and the fact that it comes in when increasing the volume on either preamp channel when the master volume is up suggests to me that this is some HF feedback.

I'd appreciate any suggestions on how to track this down and eliminate it.

I already tried different EF86 valves from JJ EF806 to a few mesh plated Mullards that I have and it's still there, so it's doesn't appear to be EF86 related.

Cheers,

Crusty
6  Muchless / DC 30 / HF squeal around 12kHz in DC30 on: April 05, 2010, 10:03:42 AM
I just tested my DC30.  Checked everything and all appears fine except for a high pitched squeal at around 12-13kHz that kicks in at around 30% volume on both the 12AX7 and EF86 channels when the master volume is >50% or bypassed.

Although it's hard to tell at the frequency, it sounds like a sine wave and the frequency shifts slightly when the volume level is changed.

Given the nature of this squeal appearing within the preamp stage and the fact that it comes in when increasing the volume on either preamp channel when the master volume is up suggests to me that this is some HF feedback.

Could anyone provide some suggestions on how to track this down and eliminate it?

I'd appreciate any suggestions.

Cheers,

Crusty
7  Website, Store / Support / HF sqeal around 12kHz in DC30 on: April 05, 2010, 10:02:35 AM
I just tested my DC30.  Checked everything and all appears fine except for a high pitched squeal at around 12-13kHz that kicks in at around 30% volume on both the 12AX7 and EF86 channels when the master volume is >50% or bypassed.

Although it's hard to tell at the frequency, it sounds like a sine wave and the frequency shifts slightly when the volume level is changed.

Given the nature of this squeal appearing within the preamp stage and the fact that it comes in when increasing the volume on either preamp channel when the master volume is up suggests to me that this is some HF feedback.

Could anyone provide some suggestions on how to track this down and eliminate it?

I'd appreciate any suggestions.

Cheers,

Crusty
8  Muchless / DC 30 / Re: Looking forward to building a couple of these on: December 04, 2009, 01:26:54 PM
Started building today.

All is very straight forward.  The first few solderings were a bit shabby (out of practice) but it soon came back to me.

The quality of the kits is great for the money with just one exception - the power indicator (jewel lamp) has to be the cheapest and nastiest one I have ever seen.  I will be getting some good quality ones with a blue jewel for the amps.

We just did a lot of the fiddly stuff today, soldering the jumper wires and resistors to the tube sockets before installing them.  I have just twisted the wires for the heaters to leave them overnight, so they stay twisted.  Also paired up and twisted the PT output cables.   So, everything is ready to install the PT and lay the heater wires.

We picked up extra cable ties to keep things neater in the amps.

My friend the luthier is a degree qualified engineer but has never really done anything much with electrical.  It's great explaining to him how a tone circuit works and he just loves the physics behind the valves.  It's awesome being able to explain a diode, triode and pentode to someone who immediately understands it.  Our valves arrived from Bob at Eurotubes and my buddy has take the GZ34 home with him just to look at it.  I remember when I enjoyed just looking at valves.  I used to love cracking open the dead ones to see the guts of them.  They really are very cool devices.

Anyway, this exercise has put me back in touch with my teenage years.   It's great getting back into electronics (analogue style), I am remembering stuff that I assumed I must have forgotten over the years.  I think back to doing work experience at school with a TV repairer (yes, back when TVs were full of valves).

I'm loving it!

Cheers,

Crusty
9  Muchless / DC 30 / Re: Looking forward to building a couple of these on: November 27, 2009, 01:17:45 AM
I have been looking at the VVR circuit and am interested in the mod.

Are you installing it in place of the master volume or as an additional control?  Are you scaling the entire amp or just the power stage.  I have only been able to find a schematic for cathode biassed amps that scales the entire amp.

I'm not sure.  If I installed the mod as an extra control, I'd probably put it on the back of the chassis.  If I was sure that it was going to perform perfectly, then I would just take the master volume control out of the circuit and put the VVR in its place.

Cheers,

Crusty
10  British Style / JCM 800 2203, 2204, 2550, 2555 / Re: GT-10 and JCM800 2203 on: November 25, 2009, 07:22:15 AM
I have an enormous amount of experience with amp modelling through Line 6 gear (PODxt Live, POD X3 Live, POD X3 Pro, Vetta II amp).

Trust me, you do not want to mix guitar amp modelling with a guitar amp.  Either use modelling and go full range flat response (direct to PA) or get your guitar amp tone from a guitar amp.

You can try putting the modelling into the effects return or power amp line in on a guitar amp (to bypass the tone stack in your guitar amp) but the tone will be shaped by your amp speaker and cabinet.  If you go this way, you will want to turn off the speaker and cabinet modelling in the GT10.\

Cheers,

Crusty
11  Muchless / DC 30 / Re: Looking forward to building a couple of these on: November 25, 2009, 07:11:14 AM
Hi Alwalt,

Hopefully the JJ EF806 tubes with the rings will do the trick.  I will be designing a soft rubber mount for the tube socket and the idea of a deflector baffle to steer the speaker backwave away from the chassis should help too.

Have you considered wiring the 2 speakers in your cab in series rather than parallel?  Two 8 Ohm speakers in series will turn your cab into a 16 Ohm cab.  Then if you add another 16 Ohm cab on the other speaker output that will give a total nominal impedance of 8 Ohms.  The trick is ensuring that the additional cab is 16 Ohms.  Wiring a 16 Ohm in parallel with something other than 16 Ohms will give some unusual nominal impedance values.

Cheers,

Crusty
12  Muchless / DC 30 / Re: Looking forward to building a couple of these on: November 21, 2009, 06:44:41 AM
The kits have arrived but I'm swamped with work for the next couple of weeks.  How irritating is that?

I have 2 research projects to finish for clients and 120 exam papers to mark from my uni students, damn!

So, it will be about 2 weeks before I can start the builds.  The maple for the cabinets is away getting sliced and dressed.  These 1x12 cabinets should look very nice.

I am looking at adding an extra baffle that will sit between the top of the back of the speaker and the chassis to reduce the back wave hitting the valves.  I am also designing a cushion mount for the EF86 (or EF806) valve.  If my design works, I won't have to contend with microphonic worries.

The cabs will be loaded with Celestion V30s (16 Ohm).  Because the speaker outputs are parallel, the 16 Ohm impedance will lend itself to adding external speaker cabinets without having the impedance drop too low in a parallel configuration.

Cheers,

Crusty

13  Muchless / DC 30 / Re: Looking forward to building a couple of these on: November 16, 2009, 05:42:11 AM
I am going to be using these too:

http://www.tuberings.com/

Cheers,

Crusty
14  Muchless / DC 30 / Re: Looking forward to building a couple of these on: November 16, 2009, 05:38:17 AM
I wonder how people with a Vox AC30 or Matchless xC30 get by.  There are a lot of combos in the world running EF86 valves that must be all be experiencing the same problem.

Cheers,

Crusty
15  Muchless / DC 30 / Re: Looking forward to building a couple of these on: November 14, 2009, 09:22:53 PM
I am probably going to get the valves from Bob at Eurotubes. What are the JJ EF86 valves like?

Cheers,

Crusty
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